mds: fix open remote dirfrag deadlock

mds: fix open remote dirfrag deadlock

During subtree migration, the importer may need to open subtree
bound dirfrags. Opening subtree bound dirfrags happens after the
exporter freeze the exporting subtee. So the discover message for
opening subtree bound dirfrags should not wait for any freezing
tree/directory, otherwise deadlock can happen.

In MDCache::handle_discover(), there are two cases can cause
discover messages wait for freezing tree/directory. One case is
fetching bare-bone dirfrags. Another case is, when merging dirfrags,
some of the dirfrags are frozen, some are freezing.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>