btrfs: replace open coded while loop with proper construct

btrfs_inc_block_group_ro wants to ensure that the current transaction is
not running dirty block groups, if it is it waits and loops again.
That logic is currently implemented using a goto label. Actually using
a proper do {} while() construct doesn't hurt readability nor does it
introduce excessive nesting and makes the relevant code stand out by
being encompassed in the loop construct. No functional changes.
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/block-group.c

@@ -2267,16 +2267,19 @@ int btrfs_inc_block_group_ro(struct btrfs_block_group *cache,
 	struct btrfs_trans_handle *trans;
 	u64 alloc_flags;
 	int ret;
+	bool dirty_bg_running;
 
-again:
+	do {
 		trans = btrfs_join_transaction(fs_info->extent_root);
 		if (IS_ERR(trans))
 			return PTR_ERR(trans);
 
+		dirty_bg_running = false;
+
 		/*
-	 * we're not allowed to set block groups readonly after the dirty
-	 * block groups cache has started writing.  If it already started,
-	 * back off and let this transaction commit
+		 * We're not allowed to set block groups readonly after the dirty
+		 * block group cache has started writing.  If it already started,
+		 * back off and let this transaction commit.
 		 */
 		mutex_lock(&fs_info->ro_block_group_mutex);
 		if (test_bit(BTRFS_TRANS_DIRTY_BG_RUN, &trans->transaction->flags)) {
@@ -2288,8 +2291,9 @@ int btrfs_inc_block_group_ro(struct btrfs_block_group *cache,
 			ret = btrfs_wait_for_commit(fs_info, transid);
 			if (ret)
 				return ret;
-		goto again;
+			dirty_bg_running = true;
 		}
+	} while (dirty_bg_running);
 
 	if (do_chunk_alloc) {
 		/*