I happened to notice this commit:
9ca415399dae - "net/mlx5: Annotate mutex destroy for root ns"
...was backported to 4.19 and 5.4 and v5.6 in linux-stable.
It patches del_sw_root_ns() - which only exists after v5.7-rc7 from:
6eb7a268a99b - "net/mlx5: Don't maintain a case of del_sw_func being null"
which creates the one line del_sw_root_ns stub function around kfree(node) by breaking it out of tree_put_node().
In the absense of del_sw_root_ns - the backport finds an identical one line kfree stub fcn - named del_sw_prio from this earlier commit:
139ed6c6c46a - "net/mlx5: Fix steering memory leak" [in v4.15-rc5]
and then puts the mutex_destroy() into that (wrong) function, instead of putting it into tree_put_node where the root ns case used to be handled.
Paul.