btrfs: calculate more accurate remaining time to sleep in transaction_kthread

If transaction_kthread is woken up before btrfs_fs_info::commit_interval
seconds have elapsed it will sleep for a fixed period of 5 seconds. This
is not a problem per-se but is not accurate. Instead the code should
sleep for an interval which guarantees on next wakeup commit_interval
would have passed. Since time tracking is not precise subtract 1 second
from delta to ensure the delay we end up waiting will be longer than
than the wake up period.
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

@@ -1794,7 +1794,9 @@ static int transaction_kthread(void *arg)
 		if (cur->state < TRANS_STATE_COMMIT_START &&
 		    delta < fs_info->commit_interval) {
 			spin_unlock(&fs_info->trans_lock);
-			delay = msecs_to_jiffies(5000);
+			delay -= msecs_to_jiffies((delta - 1) * 1000);
+			delay = min(delay,
+				    msecs_to_jiffies(fs_info->commit_interval * 1000));
 			goto sleep;
 		}
 		transid = cur->transid;