flow: fix object-size-mismatch warning in flowi{4,6}_to_flowi_common()

Commit 3df98d79 ("lsm,selinux: pass flowi_common instead of flowi
to the LSM hooks") introduced flowi{4,6}_to_flowi_common() functions which
cause UBSAN warning when building with LLVM 11.0.1 on Ubuntu 21.04.

 ================================================================================
 UBSAN: object-size-mismatch in ./include/net/flow.h:197:33
 member access within address ffffc9000109fbd8 with insufficient space
 for an object of type 'struct flowi'
 CPU: 2 PID: 7410 Comm: systemd-resolve Not tainted 5.14.0 #51
 Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 02/27/2020
 Call Trace:
  dump_stack_lvl+0x103/0x171
  ubsan_type_mismatch_common+0x1de/0x390
  __ubsan_handle_type_mismatch_v1+0x41/0x50
  udp_sendmsg+0xda2/0x1300
  ? ip_skb_dst_mtu+0x1f0/0x1f0
  ? sock_rps_record_flow+0xe/0x200
  ? inet_send_prepare+0x2d/0x90
  sock_sendmsg+0x49/0x80
  ____sys_sendmsg+0x269/0x370
  __sys_sendmsg+0x15e/0x1d0
  ? syscall_enter_from_user_mode+0xf0/0x1b0
  do_syscall_64+0x3d/0xb0
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7f7081a50497
 Code: 0c 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 89 54 24 1c 48 89 74 24 10
 RSP: 002b:00007ffc153870f8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
 RAX: ffffffffffffffda RBX: 000000000000000c RCX: 00007f7081a50497
 RDX: 0000000000000000 RSI: 00007ffc15387140 RDI: 000000000000000c
 RBP: 00007ffc15387140 R08: 0000563f29a5e4fc R09: 000000000000cd28
 R10: 0000563f29a68a30 R11: 0000000000000246 R12: 000000000000000c
 R13: 0000000000000001 R14: 0000563f29a68a30 R15: 0000563f29a5e50c
 ================================================================================

I don't think we need to call flowi{4,6}_to_flowi() from these functions
because the first member of "struct flowi4" and "struct flowi6" is

  struct flowi_common __fl_common;

while the first member of "struct flowi" is

  union {
    struct flowi_common __fl_common;
    struct flowi4       ip4;
    struct flowi6       ip6;
    struct flowidn      dn;
  } u;

which should point to the same address without access to "struct flowi".
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow.h

@@ -194,7 +194,7 @@ static inline struct flowi *flowi4_to_flowi(struct flowi4 *fl4)
 
 static inline struct flowi_common *flowi4_to_flowi_common(struct flowi4 *fl4)
 {
-	return &(flowi4_to_flowi(fl4)->u.__fl_common);
+	return &(fl4->__fl_common);
 }
 
 static inline struct flowi *flowi6_to_flowi(struct flowi6 *fl6)
@@ -204,7 +204,7 @@ static inline struct flowi *flowi6_to_flowi(struct flowi6 *fl6)
 
 static inline struct flowi_common *flowi6_to_flowi_common(struct flowi6 *fl6)
 {
-	return &(flowi6_to_flowi(fl6)->u.__fl_common);
+	return &(fl6->__fl_common);
 }
 
 static inline struct flowi *flowidn_to_flowi(struct flowidn *fldn)