hammer2 - Consolidate backend rename ops
* Reduce frontend XOP ops required for rename from 3 to 2 by integrating
the unlink-target operation into xop_nrename. The xop_nrename backend
function now handles replacing the target namespace when it exists and
will also get rid of any duplicates as a safety.
* Adjust the frontend inode locking order to try to avoid deadlocks.
* Adjust iparent documentation.
* Properly set iparent in the rename operation. The iparent was not
being adjusted at all.
* Properly set iparent in the inode create operation. The iparent was
improperly being set to 0 instead 1 when the parent directory was the
mount point.