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/ctree.h b/fs/btrfs/ctree.h
index dac6653..70c03f5 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1721,8 +1721,8 @@
 	atomic_t log_writers;
 	atomic_t log_commit[2];
 	atomic_t log_batch;
-	unsigned long log_transid;
-	unsigned long last_log_commit;
+	int log_transid;
+	int last_log_commit;
 	pid_t log_start_pid;
 	bool log_multiple_pids;