btrfs: update barrier in should_cow_block

Once there was a simple int force_cow that was used with the plain
barriers, and then converted to a bit, so we should use the appropriate
barrier helper.

Other variables in the complex if condition do not depend on a barrier,
so we should be fine in case the atomic barrier becomes a no-op.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -1441,8 +1441,8 @@ static inline int should_cow_block(struct btrfs_trans_handle *trans,
 	if (btrfs_is_testing(root->fs_info))
 		return 0;
 
-	/* ensure we can see the force_cow */
-	smp_rmb();
+	/* Ensure we can see the FORCE_COW bit */
+	smp_mb__before_atomic();
 
 	/*
 	 * We do not need to cow a block if