mds: Fix the linkmerge assert check

mds: Fix the linkmerge assert check

Let's say there is a hardlink created as below.

touch file1
ln file1 hl_file1

In this case 'file1' holds the primary inode and 'hl_file' holds
the remote inode (holds primary inode number and other required info).
Now, if the 'file1' is deleted first, it's moved to a stray directory
and can't be deleted because the hardlink 'hl_file1' still exists
which requires primary inode. So on straydn eval, this primary
inode is linked to 'hl_file1 and remote inode is removed. This is called
the linkmerge/stray reintegration. So in the linkmerge case, the srcdnl
is primary straydn and the destdnl is the remote.

Signed-off-by: Kotresh HR <khiremat@redhat.com>
Fixes: https://tracker.ceph.com/issues/61879
(cherry picked from commit 85279ac657832fd643ee2cc5f1f6a27f98e099f6)