mds: make resolve adjust dir fragmentation as needed

mds: make resolve adjust dir fragmentation as needed

During resolve, adjust dir fragmentation as needed based on the subtrees
the sender explicitly claims.  The given fragmentation on the root is
always valid.  Their bounds may not be; only split our frags as needed if
they happen to be partially in and partially out of the sender's bounding
fragset.

Signed-off-by: Sage Weil <sage@newdream.net>