4) rename() that is _not_ cross-directory. Locking rules: caller locks
the parent and finds source and target. In case of exchange (with
-RENAME_EXCHANGE in rename2() flags argument) lock both. In any case,
+RENAME_EXCHANGE in flags argument) lock both. In any case,
if the target already exists, lock it. If the source is a non-directory,
lock it. If we need to lock both, lock them in inode pointer order.
Then call the method. All locks are exclusive.