btrfs: use cond_resched_lock where possible

Clean the opencoded variant, cond_resched_lock also checks the lock for
contention so it might help in some cases that were not covered by
simple need_resched().

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 038fcf6..efc5ebf 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -93,11 +93,8 @@
 		 */
 		ASSERT(!waitqueue_active(&state->wq));
 		free_extent_state(state);
-		if (need_resched()) {
-			spin_unlock(&tree->lock);
-			cond_resched();
-			spin_lock(&tree->lock);
-		}
+
+		cond_resched_lock(&tree->lock);
 	}
 	spin_unlock(&tree->lock);
 }