mds: witness rmdir when subtrees are on other hosts

mds: witness rmdir when subtrees are on other hosts

If there is an rmdir with an empty subtree on another mds, we need to witness/
journal that on the dirfrag's auth mds so that replay correctly updates the
subtree map.

This is simpler than the rename witnesses (and the link/unlink ones) because
we aren't actually journaling a modification to any actual metadata; it's just
the subtree map that is changing.

The projection of the subtree map update needs work still, but that is also the
case for renames.

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