ceph: quota: cache inode pointer in ceph_snap_realm

ceph: quota: cache inode pointer in ceph_snap_realm

BugLink: https://bugs.launchpad.net/bugs/1875884
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>
(cherry picked from commit e3161f17d92699ce6ca3b7988131b10ad4035cf9)
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Sultan Alsawaf <sultan.alsawaf@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>