btrfs: record delta directly in transaction_kthread

Rename 'now' to 'delta' and store there the delta between transaction
start time and current time. This is in preparation for optimising the
sleep logic in the next patch. No functional changes.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1774,7 +1774,7 @@ static int transaction_kthread(void *arg)
 	struct btrfs_trans_handle *trans;
 	struct btrfs_transaction *cur;
 	u64 transid;
-	time64_t now;
+	time64_t delta;
 	unsigned long delay;
 	bool cannot_commit;
 
@@ -1790,9 +1790,9 @@ static int transaction_kthread(void *arg)
 			goto sleep;
 		}
 
-		now = ktime_get_seconds();
+		delta = ktime_get_seconds() - cur->start_time;
 		if (cur->state < TRANS_STATE_COMMIT_START &&
-		    now - cur->start_time < fs_info->commit_interval) {
+		    delta < fs_info->commit_interval) {
 			spin_unlock(&fs_info->trans_lock);
 			delay = msecs_to_jiffies(5000);
 			goto sleep;