mds/rejoin: Handle referent inode on MDSCache rejoin

mds/rejoin: Handle referent inode on MDSCache rejoin

The involves broadly following changes.

1. Add 'referent_ino' in the struct 'dn_strong' and required
   encoding/decoding of the same. Noticed that the
   MMDSCacheRejoin message actually isn't versioned yet.
   There is a tracker [1] open for it. For now, CEPH_MDS_PROTOCOL
   is bumped up as usual.

2. The following functions needs a change to construct the
   in-memory referent inode from the inode number.

    MDCache::rejoin_walk
      - add_strong_dentry, pass referent inode number to build dn_strong
    MDCache::handle_cache_rejoin_strong
      - Construct referent inode from inode number if not found in memory and add_remote_dentry
    MDCache::handle_cache_rejoin_ack
      - Bad linkage check!, construct referent inode
    MDCache::rejoin_send_acks
      - add_strong_dentry, pass referent inode number to build dn_strong
    MDCache::handle_cache_rejoin_weak
      - add_strong_dentry, pass referent inode number as 0 to build dn_strong as it's weak rejoin

[1] https://tracker.ceph.com/issues/48886

Fixes: https://tracker.ceph.com/issues/54205
Signed-off-by: Kotresh HR <khiremat@redhat.com>