[PATCH] exit: fix crash case

If we are going to BUG() not panic() here then we should cover the case of
the BUG being compiled out

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>