net/mlx4: fix some error handling in mlx4_multi_func_init()

The while loop after err_slaves should use post-decrement; otherwise
we'll fail to do the kfrees for i==0, and will run into out-of-bounds
accesses if the setup above failed already at i==0.

[I'm not sure why one even bothers populating the ->vlan_filter array:
mlx4.h isn't #included by anything outside
drivers/net/ethernet/mellanox/mlx4/, and "git grep -C2 -w vlan_filter
drivers/net/ethernet/mellanox/mlx4/" seems to suggest that the
vlan_filter elements aren't used at all.]

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/net/ethernet/mellanox/mlx4/cmd.c

@@ -2429,7 +2429,7 @@ err_thread:
 	flush_workqueue(priv->mfunc.master.comm_wq);
 	destroy_workqueue(priv->mfunc.master.comm_wq);
 err_slaves:
-	while (--i) {
+	while (i--) {
 		for (port = 1; port <= MLX4_MAX_PORTS; port++)
 			kfree(priv->mfunc.master.slave_state[i].vlan_filter[port]);
 	}