mds: Handle referent remote similar to remote

mds: Handle referent remote similar to remote

In multiple places in the code, the referent remote
needs to be handled similar to remote e.g rstats and
dirfrags check etc.

This patch adds the check in all the places so the
existing functionality doesn't break.

The changes are done in the following functions.

CDir::encode_lock_state
CDir::check_rstats
CDir::adjust_dentry_lru
CDir::steal_dentry
CDir::verify_fragstat
ScrubStack::scrub_dirfrag
Migrator::maybe_split_export
Server::reply_client_request
Server::build_snap_diff
MDCache::journal_cow_dentry
MDCache::path_traverse
MDCache::get_dentry_inode
MDCache::_open_remote_dentry_finish
MDCache::repair_dirfrag_stats_work

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