bcachefs: fix field-spanning write warning

attempts to retrofit memory safety onto C are increasingly annoying

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memcpy: detected field-spanning write (size 4) of single field "&k.replicas" at fs/bcachefs/replicas.c:454 (size 3)
WARNING: CPU: 5 PID: 6525 at fs/bcachefs/replicas.c:454 bch2_replicas_gc2+0x2cb/0x400 [bcachefs]
bch2_replicas_gc2+0x2cb/0x400:
bch2_replicas_gc2 at /home/ojab/src/bcachefs/fs/bcachefs/replicas.c:454 (discriminator 3)
Modules linked in: dm_mod tun nf_conntrack_netlink nfnetlink xt_addrtype br_netfilter overlay msr sctp bcachefs lz4hc_compress lz4_compress libcrc32c xor raid6_pq lz4_decompress pps_ldisc pps_core wireguard libchacha20poly1305 chacha_x86_64 poly1305_x86_64 ip6_udp_tunnel udp_tunnel curve25519_x86_64 libcurve25519_generic libchacha sit tunnel4 ip_tunnel af_packet bridge stp llc ip6table_nat ip6table_filter ip6_tables xt_MASQUERADE xt_conntrack iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_filter ip_tables x_tables tcp_bbr sch_fq_codel efivarfs nls_iso8859_1 nls_cp437 vfat fat cdc_mbim cdc_wdm cdc_ncm cdc_ether usbnet r8152 input_leds joydev mii amdgpu mousedev hid_generic usbhid hid ath10k_pci amd_atl edac_mce_amd ath10k_core kvm_amd ath kvm mac80211 bfq crc32_pclmul crc32c_intel polyval_clmulni polyval_generic sha512_ssse3 sha256_ssse3 sha1_ssse3 snd_hda_codec_generic snd_hda_codec_hdmi snd_hda_intel snd_intel_dspcfg i2c_algo_bit drm_exec snd_hda_codec r8169 drm_suballoc_helper
aesni_intel gf128mul crypto_simd amdxcp realtek mfd_core tpm_crb drm_buddy snd_hwdep mdio_devres libarc4 cryptd tpm_tis wmi_bmof cfg80211 evdev libphy snd_hda_core tpm_tis_core gpu_sched rapl xhci_pci xhci_hcd snd_pcm drm_display_helper snd_timer tpm sp5100_tco rfkill efi_pstore mpt3sas drm_ttm_helper ahci usbcore libaescfb ccp snd ttm 8250 libahci watchdog soundcore raid_class sha1_generic acpi_cpufreq k10temp 8250_base usb_common scsi_transport_sas i2c_piix4 hwmon video serial_mctrl_gpio serial_base ecdh_generic wmi rtc_cmos backlight ecc gpio_amdpt rng_core gpio_generic button
CPU: 5 UID: 0 PID: 6525 Comm: bcachefs Tainted: G        W          6.11.0-rc1-ojab-00058-g224bc118aec9 #6 6d5debde398d2a84851f42ab300dae32c2992027
Tainted: [W]=WARN
RIP: 0010:bch2_replicas_gc2+0x2cb/0x400 [bcachefs]
Code: c7 c2 60 91 d1 c1 48 89 c6 48 c7 c7 98 91 d1 c1 4c 89 14 24 44 89 5c 24 08 48 89 44 24 20 c6 05 fa 68 04 00 01 e8 05 a3 40 e4 <0f> 0b 4c 8b 14 24 44 8b 5c 24 08 48 8b 44 24 20 e9 55 fe ff ff 8b
RSP: 0018:ffffb434c9263d60 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff9a8efa79cc00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffb434c9263de0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000005
R13: ffff9a8efa73c300 R14: ffff9a8d9e880000 R15: ffff9a8d9e8806f8
FS:  0000000000000000(0000) GS:ffff9a9410c80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000565423373090 CR3: 0000000164e30000 CR4: 00000000003506f0
Call Trace:
<TASK>
? __warn+0x97/0x150
? bch2_replicas_gc2+0x2cb/0x400 [bcachefs 9803eca5e131ef28f26250ede34072d5b50d98b3]
bch2_replicas_gc2+0x2cb/0x400:
bch2_replicas_gc2 at /home/ojab/src/bcachefs/fs/bcachefs/replicas.c:454 (discriminator 3)
? report_bug+0x196/0x1c0
? handle_bug+0x3c/0x70
? exc_invalid_op+0x17/0x80
? __wake_up_klogd.part.0+0x4c/0x80
? asm_exc_invalid_op+0x16/0x20
? bch2_replicas_gc2+0x2cb/0x400 [bcachefs 9803eca5e131ef28f26250ede34072d5b50d98b3]
bch2_replicas_gc2+0x2cb/0x400:
bch2_replicas_gc2 at /home/ojab/src/bcachefs/fs/bcachefs/replicas.c:454 (discriminator 3)
? bch2_dev_usage_read+0xa0/0xa0 [bcachefs 9803eca5e131ef28f26250ede34072d5b50d98b3]
bch2_dev_usage_read+0xa0/0xa0:
discard_in_flight_remove at /home/ojab/src/bcachefs/fs/bcachefs/alloc_background.c:1712
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/replicas.c

@@ -451,7 +451,8 @@ int bch2_replicas_gc2(struct bch_fs *c)
 			.type = BCH_DISK_ACCOUNTING_replicas,
 		};
 
-		memcpy(&k.replicas, e, replicas_entry_bytes(e));
+		unsafe_memcpy(&k.replicas, e, replicas_entry_bytes(e),
+			      "embedded variable length struct");
 
 		struct bpos p = disk_accounting_pos_to_bpos(&k);