mm: move common segment checks to separate helper function

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Monakhov Dmitriy <dmonakhov@openvz.org>
Cc: Christoph Hellwig <hch@lst.de>
Acked-by: Anton Altaparmakov <aia21@cam.ac.uk>
Acked-by: David Chinner <dgc@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>