mds: create subtree root immediately after directory tree becomes frozen

mds: create subtree root immediately after directory tree becomes frozen

When a directory tree become frozen, its WAIT_FROZEN contexts are
executed asynchronously. Before Migrator::export_frozen() set export
bounds, MDCache::try_subtree_merge_at() can merge newly imported
subtree into the frozen directory tree. This causes problem if there
are auth pins in newly imported subtree.

The fix is creating subtree root immediately after directory tree
becomes frozen. The new subtree root has dir_auth 'me, me', so it's
not meregeable.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>