btrfs: use helpers to convert from seconds to jiffies in transaction_kthread

The kernel provides easy to understand helpers to convert from human
understandable units to the kernel-friendly 'jiffies'. So let's use
those to make the code easier to understand. 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

@@ -1780,7 +1780,7 @@ static int transaction_kthread(void *arg)
 
 	do {
 		cannot_commit = false;
-		delay = HZ * fs_info->commit_interval;
+		delay = msecs_to_jiffies(fs_info->commit_interval * 1000);
 		mutex_lock(&fs_info->transaction_kthread_mutex);
 
 		spin_lock(&fs_info->trans_lock);
@@ -1795,7 +1795,7 @@ static int transaction_kthread(void *arg)
 		    (now < cur->start_time ||
 		     now - cur->start_time < fs_info->commit_interval)) {
 			spin_unlock(&fs_info->trans_lock);
-			delay = HZ * 5;
+			delay = msecs_to_jiffies(5000);
 			goto sleep;
 		}
 		transid = cur->transid;