ceph: return -EIO if read/write against filp that lost file locks

After mds evicts session, file locks get lost sliently. It's not safe to
let programs continue to do read/write.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>