powerpc/traps: Show instructions on exceptions

Call show_user_instructions() in arch/powerpc/kernel/traps.c to dump
instructions at faulty location, useful to debugging.

Before this patch, an unhandled signal message looked like:

  pandafault[10524]: segfault (11) at 100007d0 nip 1000061c lr 7fffbd295100 code 2 in pandafault[10000000+10000]

After this patch, it looks like:

  pandafault[10524]: segfault (11) at 100007d0 nip 1000061c lr 7fffbd295100 code 2 in pandafault[10000000+10000]
  pandafault[10524]: code: 4bfffeec 4bfffee8 3c401002 38427f00 fbe1fff8 f821ffc1 7c3f0b78 3d22fffe
  pandafault[10524]: code: 392988d0 f93f0020 e93f0020 39400048 <99490000> 39200000 7d234b78 383f0040

Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>