btrfs: zoned: fix linked list corruption after log root tree allocation failure

When using a zoned filesystem, while syncing the log, if we fail to
allocate the root node for the log root tree, we are not removing the
log context we allocated on stack from the list of log contexts of the
log root tree. This means after the return from btrfs_sync_log() we get
a corrupted linked list.

Fix this by allocating the node before adding our stack allocated context
to the list of log contexts of the log root tree.

Fixes: 3ddebf27 ("btrfs: zoned: reorder log node allocation on zoned filesystem")
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NNaohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -3169,10 +3169,6 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 
 	mutex_lock(&log_root_tree->log_mutex);
 
-	index2 = log_root_tree->log_transid % 2;
-	list_add_tail(&root_log_ctx.list, &log_root_tree->log_ctxs[index2]);
-	root_log_ctx.log_transid = log_root_tree->log_transid;
-
 	if (btrfs_is_zoned(fs_info)) {
 		if (!log_root_tree->node) {
 			ret = btrfs_alloc_log_tree_node(trans, log_root_tree);
@@ -3183,6 +3179,10 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 		}
 	}
 
+	index2 = log_root_tree->log_transid % 2;
+	list_add_tail(&root_log_ctx.list, &log_root_tree->log_ctxs[index2]);
+	root_log_ctx.log_transid = log_root_tree->log_transid;
+
 	/*
 	 * Now we are safe to update the log_root_tree because we're under the
 	 * log_mutex, and we're a current writer so we're holding the commit