Clamp /debug/fail*/stacktrace-depth to MAX_STACK_TRACE_DEPTH.  Ensures that a
read of /debug/fail*/stacktrace-depth always returns a truthful answer.

Signed-off-by: Don Mullis <dwm@meer.net>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>