bcachefs: Improve backpointers error message

the error message here dated from when backpointers could be stored in
alloc keys; now, we should always print the full key.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/backpointers.c

@@ -805,7 +805,7 @@ static int check_one_backpointer(struct btree_trans *trans,
 
 	if (fsck_err_on(!k.k, c,
 			"backpointer for missing extent\n  %s",
-			(bch2_backpointer_k_to_text(&buf, c, bp.s_c), buf.buf)))
+			(bch2_bkey_val_to_text(&buf, c, bp.s_c), buf.buf)))
 		return bch2_btree_delete_at_buffered(trans, BTREE_ID_backpointers, bp.k->p);
 out:
 fsck_err: