The atomic_dec in nul_destroy() is harmless, but can trigger an underflow
error if the atomic_dec_and_test() debugging is turned on.

Problem diagnosed by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>