An #ifdef CONFIG_PPC_FPU_REGS is missing in arch_ptrace() leading
to the following Oops because [REGSET_FPR] entry is not initialised in
native_regsets[].

[   41.917608] BUG: Unable to handle kernel instruction fetch
[   41.922849] Faulting instruction address: 0xff8fd228
[   41.927760] Oops: Kernel access of bad area, sig: 11 [#1]
[   41.933089] BE PAGE_SIZE=4K PREEMPT CMPC885
[   41.940753] Modules linked in:
[   41.943768] CPU: 0 PID: 366 Comm: gdb Not tainted 5.12.0-rc5-s3k-dev-01666-g7aac86a0f057-dirty #4835
[   41.952800] NIP:  ff8fd228 LR: c004d9e0 CTR: ff8fd228
[   41.957790] REGS: caae9df0 TRAP: 0400   Not tainted  (5.12.0-rc5-s3k-dev-01666-g7aac86a0f057-dirty)
[   41.966741] MSR:  40009032 <EE,ME,IR,DR,RI>  CR: 82004248  XER: 20000000
[   41.973540]
[   41.973540] GPR00: c004d9b4 caae9eb0 c1b64f60 c1b64520 c0713cd4 caae9eb8 c1bacdfc 00000004
[   41.973540] GPR08: 00000200 ff8fd228 c1bac700 00001032 28004242 1061aaf4 00000001 106d64a0
[   41.973540] GPR16: 00000000 00000000 7fa0a774 10610000 7fa0aef9 00000000 10610000 7fa0a538
[   41.973540] GPR24: 7fa0a580 7fa0a570 c1bacc00 c1b64520 c1bacc00 caae9ee8 00000108 c0713cd4
[   42.009685] NIP [ff8fd228] 0xff8fd228
[   42.013300] LR [c004d9e0] __regset_get+0x100/0x124
[   42.018036] Call Trace:
[   42.020443] [caae9eb0] [c004d9b4] __regset_get+0xd4/0x124 (unreliable)
[   42.026899] [caae9ee0] [c004da94] copy_regset_to_user+0x5c/0xb0
[   42.032751] [caae9f10] [c002f640] sys_ptrace+0xe4/0x588
[   42.037915] [caae9f30] [c0011010] ret_from_syscall+0x0/0x28
[   42.043422] --- interrupt: c00 at 0xfd1f8e4
[   42.047553] NIP:  0fd1f8e4 LR: 1004a688 CTR: 00000000
[   42.052544] REGS: caae9f40 TRAP: 0c00   Not tainted  (5.12.0-rc5-s3k-dev-01666-g7aac86a0f057-dirty)
[   42.061494] MSR:  0000d032 <EE,PR,ME,IR,DR,RI>  CR: 48004442  XER: 00000000
[   42.068551]
[   42.068551] GPR00: 0000001a 7fa0a040 77dad7e0 0000000e 00000170 00000000 7fa0a078 00000004
[   42.068551] GPR08: 00000000 108deb88 108dda40 106d6010 44004442 1061aaf4 00000001 106d64a0
[   42.068551] GPR16: 00000000 00000000 7fa0a774 10610000 7fa0aef9 00000000 10610000 7fa0a538
[   42.068551] GPR24: 7fa0a580 7fa0a570 1078fe00 1078fd70 1078fd70 00000170 0fdd3244 0000000d
[   42.104696] NIP [0fd1f8e4] 0xfd1f8e4
[   42.108225] LR [1004a688] 0x1004a688
[   42.111753] --- interrupt: c00
[   42.114768] Instruction dump:
[   42.117698] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
[   42.125443] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
[   42.133195] ---[ end trace d35616f22ab2100c ]---

Adding the missing #ifdef is not good because gdb doesn't like getting
an error when getting registers.

Instead, make ptrace return 0s when CONFIG_PPC_FPU_REGS is not set.

Fixes: b6254ced ("powerpc/signal: Don't manage floating point regs when no FPU")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9121a44a2d50ba1af18d8aa5ada06c9a3bea8afd.1617200085.git.christophe.leroy@csgroup.eu