Btrfs: fix extent_buffer leak during tree log replay

During tree log replay, we read in the tree log roots,
process them and then free them.  A recent change
takes an extra reference on the root node of the tree
when the root is read in, and stores that reference
in root->commit_root.

This reference was not being freed, leaving us with
one buffer pinned in ram for each subvol with
a tree log root after a crash.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/tree-log.c

@@ -3019,6 +3019,7 @@ again:
 		key.offset = found_key.offset - 1;
 		wc.replay_dest->log_root = NULL;
 		free_extent_buffer(log->node);
+		free_extent_buffer(log->commit_root);
 		kfree(log);
 
 		if (found_key.offset == 0)