bcachefs: Apply workaround for too many btree iters to read path

Reading from cached data, which calls bch2_bucket_io_time_reset(), is
leading to transaction iterator overflows - this standardizes the
workaround.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_iter.h

@@ -328,13 +328,19 @@ static inline struct bkey_s_c __bch2_btree_iter_peek(struct btree_iter *iter,
 		: bch2_btree_iter_peek(iter);
 }
 
+static inline int btree_trans_too_many_iters(struct btree_trans *trans)
+{
+	return hweight64(trans->paths_allocated) > BTREE_ITER_MAX / 2
+		? -EINTR : 0;
+}
+
 static inline struct bkey_s_c
 __bch2_btree_iter_peek_and_restart(struct btree_trans *trans,
 				   struct btree_iter *iter, unsigned flags)
 {
 	struct bkey_s_c k;
 
-	while ((hweight64(trans->paths_allocated) > BTREE_ITER_MAX / 2) ||
+	while (btree_trans_too_many_iters(trans) ||
 	       (k = __bch2_btree_iter_peek(iter, flags),
 		bkey_err(k) == -EINTR))
 		bch2_trans_begin(trans);

fs/bcachefs/dirent.c

@@ -531,11 +531,10 @@ int bch2_readdir(struct bch_fs *c, subvol_inum inum, struct dir_context *ctx)
 		 * read_target looks up subvolumes, we can overflow paths if the
 		 * directory has many subvolumes in it
 		 */
-		if (hweight64(trans.paths_allocated) > BTREE_ITER_MAX / 2) {
-			ret = -EINTR;
+		ret = btree_trans_too_many_iters(&trans);
+		if (ret)
 			break;
 	}
-	}
 	bch2_trans_iter_exit(&trans, &iter);
 err:
 	if (ret == -EINTR)

fs/bcachefs/fs-io.c

@@ -881,6 +881,10 @@ static void bchfs_read(struct btree_trans *trans,
 
 		swap(rbio->bio.bi_iter.bi_size, bytes);
 		bio_advance(&rbio->bio, bytes);
+
+		ret = btree_trans_too_many_iters(trans);
+		if (ret)
+			break;
 	}
 err:
 	bch2_trans_iter_exit(trans, &iter);

fs/bcachefs/io.c

@@ -2395,6 +2395,10 @@ void __bch2_read(struct bch_fs *c, struct bch_read_bio *rbio,
 
 		swap(bvec_iter.bi_size, bytes);
 		bio_advance_iter(&rbio->bio, &bvec_iter, bytes);
+
+		ret = btree_trans_too_many_iters(&trans);
+		if (ret)
+			break;
 	}
 err:
 	bch2_trans_iter_exit(&trans, &iter);