mds: Hardlink referent inode plumbing work

mds: Hardlink referent inode plumbing work

The linkage_t struct changes:
  - Add referent_ino(inodeno_t) and referent_inode(CInode *) to the
    linkage_t. These two fields becomes the core and identifies the
    linkage as referent remote.
  - Add 'is_referent_remote()' to check if it's referent remote
    and modify 'is_remote()', 'is_null()' check accordingly.
  - Add functions to access these fields.

Add the following new functions to link referent inode.
  1. CDentry::push_projected_linkage - with referent_inode
  2. CDir::link_null_referent_inode
  3. CDir::link_referent_inode

Modify the following existing functions to link/unlink referent inode
  1. CDentry::link_remote
  2. CDentry::unlink_remote
  3. CDentry::pop_projected_linakge
  4. CDentry::CDentry
  5. CDir::add_remote_dentry
  6. CDir::link_inode_work
  7. CDir::unlink_inode_work

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