ceph: align data in pages in ceph_sync_write

Encrypted files will need to be dealt with in block-sized chunks and
once we do that, the way that ceph_sync_write aligns the data in the
bounce buffer won't be acceptable.

Change it to align the data the same way it would be aligned in the
pagecache.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>