md/md-multipath: enable io accounting

use md_account_bio() to enable io accounting, also make sure
mddev_suspend() will wait for all io to be done.

Signed-off-by: Yu Kuai <[e-mail]>
Reviewed-by: Xiao Ni <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/md/md-multipath.c

···
 	    && md_flush_request(mddev, bio))
 		return true;
 
+	md_account_bio(mddev, &bio);
 	mp_bh = mempool_alloc(&conf->pool, GFP_NOIO);
 
 	mp_bh->master_bio = bio;