mds: close exported dirfrag

mds: close exported dirfrag

We have to keep export bounds open for auth subtrees.  After we export a
subtree, though, there are two opportunities to drop empty dirfrags from
our cache:

 - The children of the exported subtree may now be trimmable, if they are
   also non-auth and empty.
 - The exported subtree may be trimmable if it is empty and the parent is
   also non-auth.  This may be true for ancestors further up the hierarchy
   as well.

This helps ensure that when we get to rejoin, the only non-auth subtrees we
have are there because they are non-empty or because they are bounds on our
own subtrees.

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