Btrfs: use signed integer instead of unsigned long integer for log transid

The log trans id is initialized to be 0 every time we create a log tree,
and the log tree need be re-created after a new transaction is started,
it means the log trans id is unlikely to be a huge number, so we can use
signed integer instead of unsigned long integer to save a bit space.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 285c168..128a904 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2362,7 +2362,7 @@
 }
 
 static int wait_log_commit(struct btrfs_trans_handle *trans,
-			   struct btrfs_root *root, unsigned long transid)
+			   struct btrfs_root *root, int transid)
 {
 	DEFINE_WAIT(wait);
 	int index = transid % 2;
@@ -2434,7 +2434,7 @@
 	int ret;
 	struct btrfs_root *log = root->log_root;
 	struct btrfs_root *log_root_tree = root->fs_info->log_root_tree;
-	unsigned long log_transid = 0;
+	int log_transid = 0;
 	struct blk_plug plug;
 
 	mutex_lock(&root->log_mutex);