md: fix memleak for mempool

I noticed kmemleak report memory leak when run create/stop
md in a loop, backtrace:
[<000000001ca975e7>] mempool_create_node+0x86/0xd0
[<0000000095576bcd>] md_run+0x1057/0x1410 [md_mod]
[<000000007b45c5fc>] do_md_run+0x15/0x130 [md_mod]
[<000000001ede9ec0>] md_ioctl+0x1f49/0x25d0 [md_mod]
[<000000004142cacf>] blkdev_ioctl+0x680/0xd00

The root cause is we alloc mddev->flush_pool and
mddev->flush_bio_pool in md_run, but from do_md_stop
will not call into md_stop but __md_stop, move the
mempool_destroy to __md_stop fixes the problem for me.

The bug was introduced in 5a409b4f56d5, the fixes should go to
4.18+

Fixes: 5a409b4f56d5 ("MD: fix lock contention for flush bios")
Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -5904,14 +5904,6 @@ static void __md_stop(struct mddev *mddev)
 		mddev->to_remove = &md_redundancy_group;
 	module_put(pers->owner);
 	clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
-}
-
-void md_stop(struct mddev *mddev)
-{
-	/* stop the array and free an attached data structures.
-	 * This is called from dm-raid
-	 */
-	__md_stop(mddev);
 	if (mddev->flush_bio_pool) {
 		mempool_destroy(mddev->flush_bio_pool);
 		mddev->flush_bio_pool = NULL;
@@ -5920,6 +5912,14 @@ void md_stop(struct mddev *mddev)
 		mempool_destroy(mddev->flush_pool);
 		mddev->flush_pool = NULL;
 	}
+}
+
+void md_stop(struct mddev *mddev)
+{
+	/* stop the array and free an attached data structures.
+	 * This is called from dm-raid
+	 */
+	__md_stop(mddev);
 	bioset_exit(&mddev->bio_set);
 	bioset_exit(&mddev->sync_set);
 }