mds: fix race between submitting EImportFinish and SubtreeMap

mds: fix race between submitting EImportFinish and SubtreeMap

MDCache::create_subtree_map() use MDCache::my_ambiguous_imports
and Migrator::is_ambiguous_import() to decide if a subtree is
ambiguous import.  Submitting log event can start new segment
and submit an extra SubtreeMap. So before submitting EImportFinish
event, we need to cleanup MDCache::my_ambiguous_imports and
Migrator::import_state.

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