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>
try_remove_dentries_for_stray() remove dirty dentries and marks dir clean. It means the on-disk dirfrag object is stale. Fetching it may add wrong dentries to the dir.