mds: refragment dirs when inode dirfragtree updates from journal

mds: refragment dirs when inode dirfragtree updates from journal

Force dir fragmentation specified by dirfragtree when replayed from
the journal.

Example:
 mds0 is auth for /foo, mds1 is auth for /foo/bar.
 mds1 fragments /foo/bar.  journals etc.
 mds0 gets fragment notify and the in-memory inode's dirfragtree changes.
 mds0 journals the /foo/bar inode for some random reason.
 mds0 imports /foo/bar.

On replay, mds0 refragments upon first mention of the new fragtree in the
journal, so that the dirfragtree <-> dir frags always match.  Confusion is
avoided when we, say, import /foo/bar.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>