fault_inject: silence a warning

lib/fault-inject.c:168: warning: 'debugfs_create_ul_MAX_STACK_TRACE_DEPTH' defined but not used

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>