mds: enqueue ~mdsdir at the time of enqueing root

mds: enqueue ~mdsdir at the time of enqueing root

This would avoid the need to run individual scrubs for
~mdsdir and root, i.e. run both the scrubs under the
same header, this also helps to avoid edge case where
in case ~mdsdir is huge and it's taking time to scrub it,
the scrub status would report something like this until
root inodes kick in:

{
    "status": "scrub active (757 inodes in the stack)",
    "scrubs": {}
}

Fixes: https://tracker.ceph.com/issues/59350
Signed-off-by: Dhairya Parmar <dparmar@redhat.com>
(cherry picked from commit f85b22818b474911cfdc15e7c6be1d2979939888)