kcov: ignore fault-inject and stacktrace

Don't instrument 3 more files that contain debugging facilities and
produce large amounts of uninteresting coverage for every syscall.

The following snippets are sprinkled all over the place in kcov traces
in a debugging kernel.  We already try to disable instrumentation of
stack unwinding code and of most debug facilities.  I guess we did not
use fault-inject.c at the time, and stacktrace.c was somehow missed (or
something has changed in kernel/configs).  This change both speeds up
kcov (kernel doesn't need to store these PCs, user-space doesn't need to
process them) and frees trace buffer capacity for more useful coverage.

  should_fail
  lib/fault-inject.c:149
  fail_dump
  lib/fault-inject.c:45

  stack_trace_save
  kernel/stacktrace.c:124
  stack_trace_consume_entry
  kernel/stacktrace.c:86
  stack_trace_consume_entry
  kernel/stacktrace.c:89
  ... a hundred frames skipped ...
  stack_trace_consume_entry
  kernel/stacktrace.c:93
  stack_trace_consume_entry
  kernel/stacktrace.c:86

Link: http://lkml.kernel.org/r/20200116111449.217744-1-dvyukov@gmail.com
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>