mlx4core: Fix a memory leak when deleting slave's resources

mlx4_delete_all_resources_for_slave() in the resource tracker should free
all memory allocated for a slave. While releasing memory of fs_rule,
it misses releasing memory of fs_rule->mirr_mbox.

Linux commit:
461d5f1b59490ce0096dfda45e10038c122a7892

PR:		264249
MFC after:	1 week
Sponsored by:	NVIDIA Networking

sys/dev/mlx4/mlx4_core/mlx4_resource_tracker.c

@@ -5063,6 +5063,7 @@ static void rem_slave_fs_rule(struct mlx4_dev *dev, int slave)
 						 &tracker->res_tree[RES_FS_RULE]);
 					list_del(&fs_rule->com.list);
 					spin_unlock_irq(mlx4_tlock(dev));
+					kfree(fs_rule->mirr_mbox);
 					kfree(fs_rule);
 					state = 0;
 					break;