btrfs: error when COWing block from a root that is being deleted

At btrfs_cow_block() we check if the block being COWed belongs to a root
that is being deleted and if so we log an error message. However this is
an unexpected case and it indicates a bug somewhere, so we should return
an error and abort the transaction. So change this in the following ways:

1) Abort the transaction with -EUCLEAN, so that if the issue ever happens
   it can easily be noticed;

2) Change the logged message level from error to critical, and change the
   message itself to print the block's logical address and the ID of the
   root;

3) Return -EUCLEAN to the caller;

4) As this is an unexpected scenario, that should never happen, mark the
   check as unlikely, allowing the compiler to potentially generate better
   code.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -682,9 +682,13 @@ noinline int btrfs_cow_block(struct btrfs_trans_handle *trans,
 	u64 search_start;
 	int ret;
 
-	if (test_bit(BTRFS_ROOT_DELETING, &root->state))
-		btrfs_err(fs_info,
-			"COW'ing blocks on a fs root that's being dropped");
+	if (unlikely(test_bit(BTRFS_ROOT_DELETING, &root->state))) {
+		btrfs_abort_transaction(trans, -EUCLEAN);
+		btrfs_crit(fs_info,
+		   "attempt to COW block %llu on root %llu that is being deleted",
+			   buf->start, btrfs_root_id(root));
+		return -EUCLEAN;
+	}
 
 	/*
 	 * COWing must happen through a running transaction, which always