btrfs: reduce critical section at btrfs_wait_ordered_extents()

At btrfs_wait_ordered_extents(), there's no point in updating the counters
after locking the root's ordered extent lock, as the counters are local.
So change this to update the counters before taking the lock.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ordered-data.c

@@ -783,10 +783,10 @@ u64 btrfs_wait_ordered_extents(struct btrfs_root *root, u64 nr,
 		btrfs_queue_work(fs_info->flush_workers, &ordered->flush_work);
 
 		cond_resched();
-		spin_lock(&root->ordered_extent_lock);
 		if (nr != U64_MAX)
 			nr--;
 		count++;
+		spin_lock(&root->ordered_extent_lock);
 	}
 	list_splice_tail(&skipped, &root->ordered_extents);
 	list_splice_tail(&splice, &root->ordered_extents);