md/raid1: perform mem allocation before disabling writes during resync.

Though this mem alloc is GFP_NOIO an so will not deadlock, it seems
better to do the allocation before 'raise_barrier' which stops any IO
requests while the resync proceeds.

raid10 always uses this order, so it is at least consistent to do the
same in raid1.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid1.c

@@ -1749,11 +1749,11 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
 		msleep_interruptible(1000);
 
 	bitmap_cond_end_sync(mddev->bitmap, sector_nr);
+	r1_bio = mempool_alloc(conf->r1buf_pool, GFP_NOIO);
 	raise_barrier(conf);
 
 	conf->next_resync = sector_nr;
 
-	r1_bio = mempool_alloc(conf->r1buf_pool, GFP_NOIO);
 	rcu_read_lock();
 	/*
 	 * If we get a correctably read error during resync or recovery,