Use the macro shm_ids().

Remove useless check for a userspace pointer, because copy_to_user()
will check it.

Some style cleanups.
Signed-off-by: NWANG Cong <wangcong@zeuux.org>
Cc: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>