ceph: quota: cache inode pointer in ceph_snap_realm

Keep a pointer to the inode in struct ceph_snap_realm.  This allows to
optimize functions that walk the realms hierarchy (e.g. in quotas).

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