ceph: use release_pages() directly

release_pages() has been available to modules since Oct, 2010,
when commit 0be8557b

 ("fuse: use release_pages()") added
EXPORT_SYMBOL(release_pages). However, this ceph code was still
using a workaround.

Remove the workaround, and call release_pages() directly.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>