x86/traps: Factor out extable_fixup() and make printing consistent

x86/traps: Factor out extable_fixup() and make printing consistent

UD faults never had any diagnostics printed, and the others were inconsistent.

Don't use dprintk() because identifying traps.c is actively unhelpful in the
message, as it is the location of the fixup, not the fault.  Use the new
vec_name() infrastructure, rather than leaving raw numbers for the log.

  (XEN) Running stub recovery selftests...
  (XEN) Fixup #UD[0000]: ffff82d07fffd040 [ffff82d07fffd040] -> ffff82d0403ac9d6
  (XEN) Fixup #GP[0000]: ffff82d07fffd041 [ffff82d07fffd041] -> ffff82d0403ac9d6
  (XEN) Fixup #SS[0000]: ffff82d07fffd040 [ffff82d07fffd040] -> ffff82d0403ac9d6
  (XEN) Fixup #BP[0000]: ffff82d07fffd041 [ffff82d07fffd041] -> ffff82d0403ac9d6

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>