ceph: clean up ceph.dir.pin vxattr name sizeof()

.name_size should use the same string as .name.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>