filestore: set guard on collection_move

filestore: set guard on collection_move

During recovery we submit transactions like:

 - delete a/foo
 - move tmp/foo to a/foo

This prevents the EEXIST check in collection_move from doing any good,
since the destination never exists.  We need to do that remove at least
sometimes, because we may be overwriting an existing/older version of the
object.

So,
 - set the guard after we do the move, so that
 - the delete won't be repated, and
 - the EEXIST check will work

Also check the guard for good measure (although that doesn't do anything
specifically useful in this scenario).

Fixes: #2164
Signed-off-by: Sage Weil <sage@newdream.net>
Reviewed-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Samuel Just <samuel.just@dreamhost.com>