fix undefined reference to user_shm_unlock

fix undefined reference to user_shm_unlock

commit 2195d2818c37bdf263865f1e9effccdd9fc5f9d4 upstream.

My 353d5c30c666580347515da609dd74a2b8e9b828 "mm: fix hugetlb bug due to
user_shm_unlock call" broke the CONFIG_SYSVIPC !CONFIG_MMU build of both
2.6.31 and 2.6.30.6: "undefined reference to `user_shm_unlock'".

gcc didn't understand my comment! so couldn't figure out to optimize
away user_shm_unlock() from the error path in the hugetlb-less case, as
it does elsewhere.  Help it to do so, in a language it understands.

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>