mds: drop dirty dentries in deleted directory

opened dirfrags and null dirty dentries in deleted directory inode
prevent MDCache::eval_stray() from purging the delete inode.

It's safe to not commit null dirty dentries in deleted directory to
corresponding dirfrag objects, because these dirfrag objects will be
deleted soon.

Fixes: #10164
Signed-off-by: Yan, Zheng <zyan@redhat.com>
I was worry about race between fetching dirfrag and unlinking directory inode. I think about this again, the race shouldn't happen. Will remove this code.