ceph: return the real size read when it hits EOF

Currently, if the sync read handler ends up reading more from the last
object in the file than the i_size indicates, then it'll end up
returning the wrong length. Ensure that we cap the returned length and
pos at the EOF.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>