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 5a409b4f, the fixes should go to
4.18+

Fixes: 5a409b4f

 ("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>