btrfs: zoned: move log tree node allocation out of log_root_tree->log_mutex

Commit 6e37d245 ("btrfs: zoned: fix deadlock on log sync") pointed out
a deadlock warning and removed mutex_{lock,unlock} of fs_info::tree_root->log_mutex.
While it looks like it always cause a deadlock, we didn't see actual
deadlock in fstests runs. The reason is log_root_tree->log_mutex !=
fs_info->tree_root->log_mutex, not taking the same lock. So, the warning
was actually a false-positive.

Since btrfs_alloc_log_tree_node() is protected only by
fs_info->tree_root->log_mutex, we can (and should) move the code out of
the lock scope of log_root_tree->log_mutex and silence the warning.

Fixes: 6e37d245 ("btrfs: zoned: fix deadlock on log sync")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -3165,20 +3165,22 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	 */
 	mutex_unlock(&root->log_mutex);
 
-	btrfs_init_log_ctx(&root_log_ctx, NULL);
-
-	mutex_lock(&log_root_tree->log_mutex);
-
 	if (btrfs_is_zoned(fs_info)) {
+		mutex_lock(&fs_info->tree_root->log_mutex);
 		if (!log_root_tree->node) {
 			ret = btrfs_alloc_log_tree_node(trans, log_root_tree);
 			if (ret) {
-				mutex_unlock(&log_root_tree->log_mutex);
+				mutex_unlock(&fs_info->tree_log_mutex);
 				goto out;
 			}
 		}
+		mutex_unlock(&fs_info->tree_root->log_mutex);
 	}
 
+	btrfs_init_log_ctx(&root_log_ctx, NULL);
+
+	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;