Slight optimization possible in fsavl_create

fsavl_create is part of the ZFS receive code path. It builds up an AVL tree of filesystem information from the replication stream. In one part it checks if an element already exists in the tree and adds it if not, but does so in a way that searches the tree again to ensure we are not inserting a duplicate entry, which we already know not to be the case. This can be trivially optimized to only do the search once.