ceph: pass ino# instead of old_dentry if it's disconnected

When exporting the kceph to NFS it may pass a DCACHE_DISCONNECTED
dentry for the link operation. Then it will parse this dentry as a
snapdir, and the mds will fail the link request as -EROFS.

MDS allow clients to pass a ino# instead of a path.

Link: https://tracker.ceph.com/issues/59515


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>