mds: keep dentry lock in sync state

mds: keep dentry lock in sync state

unlike locks of other types, dentry lock in unreadable state can
block path traverse, so it should be in sync state as much as
possible.

This patch make Locker::try_eval() change dentry lock's state to
sync even when the dentry is freezing. Also make migrator check
imported dentries' lock states, change locks' states to sync if
necessary.

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