net/mlx4_core: Add missed mlx4_free_cmd_mailbox()

mlx4_do_mirror_rule() forgets to call mlx4_free_cmd_mailbox() to
free the memory region allocated by mlx4_alloc_cmd_mailbox() before
an exit.
Add the missed call to fix it.

Fixes: 78efed2 ("net/mlx4_core: Support mirroring VF DMFS rules on both ports")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20210221143559.390277-1-hslester96@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -4986,6 +4986,7 @@ static int mlx4_do_mirror_rule(struct mlx4_dev *dev, struct res_fs_rule *fs_rule
 
 	if (!fs_rule->mirr_mbox) {
 		mlx4_err(dev, "rule mirroring mailbox is null\n");
+		mlx4_free_cmd_mailbox(dev, mailbox);
 		return -EINVAL;
 	}
 	memcpy(mailbox->buf, fs_rule->mirr_mbox, fs_rule->mirr_mbox_size);