ipc/shm: some shmat cleanups

Clean up early flag and address some minutia.

Link: http://lkml.kernel.org/r/1486673582-6979-3-git-send-email-dave@stgolabs.net


Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>