bcachefs: Check for read_time == 0 in bch2_alloc_v4_invalid()

We've been seeing this error in fsck and we weren't able to track down
where it came from - but now that .key_invalid methods take a rw
argument, we can safely check for this.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/alloc_background.c

@@ -346,12 +346,23 @@ int bch2_alloc_v3_invalid(const struct bch_fs *c, struct bkey_s_c k,
 int bch2_alloc_v4_invalid(const struct bch_fs *c, struct bkey_s_c k,
 			  int rw, struct printbuf *err)
 {
+	struct bkey_s_c_alloc_v4 a = bkey_s_c_to_alloc_v4(k);
+
 	if (bkey_val_bytes(k.k) != sizeof(struct bch_alloc_v4)) {
 		pr_buf(err, "bad val size (%zu != %zu)",
 		       bkey_val_bytes(k.k), sizeof(struct bch_alloc_v4));
 		return -EINVAL;
 	}
 
+	if (rw == WRITE) {
+		if (a.v->cached_sectors &&
+		    !a.v->dirty_sectors &&
+		    !a.v->io_time[READ]) {
+			pr_buf(err, "cached bucket with read_time == 0");
+			return -EINVAL;
+		}
+	}
+
 	return 0;
 }