mds: don't journal inode when merging subtrees

mds: don't journal inode when merging subtrees

When doing subtree migration or cross subtree rename, we rdlock
fragtreelock of subtree boundaries' inodes. rdlock fragtreelock
has already journaled dirfragtree updates. I can't see why we
need to journal it again when merging subtrees.

The resaon I want to remove this code is that it can journal
unconnected dirfrag in the Migrator::export_reverse() case.

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