mds: skip sr moves when target is an unlinked dir

mds: skip sr moves when target is an unlinked dir

A directory in the stray directory cannot have any HEAD inodes with caps so
there is no need to move anything to the snaprealm opened for the unlinked
directory.

Following the parent commit's reproducer, the behavior now looks expectedly like:

    2024-02-27T02:26:59.049+0000 7f5b095f3700 10 mds.0.cache.ino(0x100000f4575) open_snaprealm snaprealm(0x100000f4575 seq 0 lc 0 cr 0 cps 1 snaps={} last_modified 0.000000 change_attr 0 0x5632a57f9680) parent is snaprealm(0x1 seq e lc e cr e cps 1 snaps={2=snap(2 0x1 'one' 2024-02-27T01:34:36.001053+0000),3=snap(3 0x1 'two' 2024-02-27T01:34:48.623349+0000),6=snap(6 0x1 'six' 2024-02-27T02:03:51.619896+0000),7=snap(7 0x1 'seven' 2024-02-27T02:04:28.375336+0000),8=snap(8 0x1 '1' 2024-02-27T02:06:14.170884+0000),9=snap(9 0x1 '2' 2024-02-27T02:09:47.158624+0000),a=snap(a 0x1 '3' 2024-02-27T02:18:24.666934+0000),b=snap(b 0x1 '4' 2024-02-27T02:18:38.268874+0000),c=snap(c 0x1 '5' 2024-02-27T02:23:13.183995+0000),d=snap(d 0x1 '6' 2024-02-27T02:25:25.593014+0000),e=snap(e 0x1 '7' 2024-02-27T02:26:55.184945+0000)} last_modified 2024-02-27T02:26:55.184945+0000 change_attr 11 0x5632861c5680)
    2024-02-27T02:26:59.049+0000 7f5b095f3700 10  mds.0.cache.snaprealm(0x1 seq 14 0x5632861c5680) split_at: snaprealm(0x100000f4575 seq 0 lc 0 cr 0 cps 1 snaps={} last_modified 0.000000 change_attr 0 0x5632a57f9680) on [inode 0x100000f4575 [...f,head] ~mds0/stray0/100000f4575/ auth v1199 pv1200 ap=3 snaprealm=0x5632a57f9680 DIRTYPARENT f() n(v0 1=0+1) old_inodes=1 (ilink xlockdone x=1) (isnap xlockdone x=1) (inest lock) (ifile excl) (iversion lock w=1 last_client=4365) caps={4365=pAsLsXsFsx/AsLsXsFsx@6},l=4365 | request=1 lock=3 dirfrag=1 caps=1 dirtyparent=1 dirty=1 waiter=0 authpin=1 0x563385e94000]
    2024-02-27T02:26:59.049+0000 7f5b095f3700 10  mds.0.cache.snaprealm(0x1 seq 14 0x5632861c5680)  moving unlinked directory inode

Discussions with Dan van der Ster led to the creation of this patch.

Fixes: https://tracker.ceph.com/issues/53192
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
Signed-off-by: Dan van der Ster <dan.vanderster@clyso.com>
(cherry picked from commit c190a3f1633e9282772e5ec54fe10556856a2540)