ceph: fix infinite loop in get_quota_realm()

get_quota_realm() enters infinite loop if quota inode has no caps.
This can happen after client gets evicted.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Luis Henriques <lhenriques@suse.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>