btrfs: consolidate btrfs_previous_item ret val handling in btrfs_shrink_device

Instead of having three 'if' to handle non-NULL return value consolidate
this in one 'if (ret)'. That way the code is more obvious:

 - Always drop delete_unused_bgs_mutex if ret is not NULL
 - If ret is negative -> goto done
 - If it's 1 -> reset ret to 0, release the path and finish the loop.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -4668,11 +4668,10 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
 		}
 
 		ret = btrfs_previous_item(root, path, 0, key.type);
-		if (ret)
+		if (ret) {
 			mutex_unlock(&fs_info->delete_unused_bgs_mutex);
 			if (ret < 0)
 				goto done;
-		if (ret) {
 			ret = 0;
 			btrfs_release_path(path);
 			break;