bcachefs: Improve error message when finding wrong btree node

single_device.merge_torture_flakey is, very rarely, finding a btree node
that doesn't match the key that points to it: this patch improves the
error message to print out more fields from the btree node header, so
that we can see what else does or does not match the key.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_io.c

@@ -968,12 +968,20 @@ int bch2_btree_node_read_done(struct bch_fs *c, struct bch_dev *ca,
 		struct bch_btree_ptr_v2 *bp =
 			&bkey_i_to_btree_ptr_v2(&b->key)->v;
 
+		bch2_bpos_to_text(&buf, b->data->min_key);
+		prt_str(&buf, "-");
+		bch2_bpos_to_text(&buf, b->data->max_key);
+
 		btree_err_on(b->data->keys.seq != bp->seq,
 			     -BCH_ERR_btree_node_read_err_must_retry,
 			     c, ca, b, NULL,
 			     btree_node_bad_seq,
-			     "got wrong btree node (seq %llx want %llx)",
-			     b->data->keys.seq, bp->seq);
+			     "got wrong btree node (want %llx got %llx)\n"
+			     "got btree %s level %llu pos %s",
+			     bp->seq, b->data->keys.seq,
+			     bch2_btree_id_str(BTREE_NODE_ID(b->data)),
+			     BTREE_NODE_LEVEL(b->data),
+			     buf.buf);
 	} else {
 		btree_err_on(!b->data->keys.seq,
 			     -BCH_ERR_btree_node_read_err_must_retry,