iov_iter_get_pages{,_alloc}(): cap the maxsize with MAX_RW_COUNT

All callers can and should handle iov_iter_get_pages() returning
fewer pages than requested.  All in-kernel ones do.  And it makes
the arithmetical overflow analysis much simpler...

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>