mds: clear EXPORTBOUND before try merging subtree

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: don't replicate purging dentry

open_remote_ino is racy, it's possible someone deletes the inode's
last linkage while the MDS is discovering the inode.

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