bcachefs: Some fixes for building in userspace

userspace allocators don't align allocations as nicely as kernel
allocators, which meant that in some cases we weren't allocating big
enough bvec arrays - just make the calculations more rigorous and
explicit to fix it.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bset.c

@@ -1672,7 +1672,7 @@ struct bkey_packed *bch2_btree_node_iter_prev_filter(struct btree_node_iter *ite
 	struct bkey_packed *orig_pos = bch2_btree_node_iter_peek_all(iter, b);
 	struct btree_node_iter_set *set;
 	struct bset_tree *t;
-	unsigned end;
+	unsigned end = 0;
 
 	bch2_btree_node_iter_verify(iter, b);
 

fs/bcachefs/btree_io.c

@@ -1881,7 +1881,8 @@ void __bch2_btree_node_write(struct bch_fs *c, struct btree *b,
 
 	trace_btree_write(b, bytes_to_write, sectors_to_write);
 
-	wbio = container_of(bio_alloc_bioset(NULL, 1 << order,
+	wbio = container_of(bio_alloc_bioset(NULL,
+				buf_pages(data, sectors_to_write << 9),
 				REQ_OP_WRITE|REQ_META|REQ_FUA,
 				GFP_NOIO,
 				&c->btree_bio),

fs/bcachefs/journal_io.c

@@ -429,7 +429,6 @@ static int journal_read_bucket(struct bch_dev *ca,
 {
 	struct bch_fs *c = ca->fs;
 	struct journal_device *ja = &ca->journal;
-	struct bio *bio = ja->bio;
 	struct jset *j = NULL;
 	unsigned sectors, sectors_read = 0;
 	u64 offset = bucket_to_sector(ca, ja->buckets[bucket]),
@@ -441,15 +440,22 @@ static int journal_read_bucket(struct bch_dev *ca,
 
 	while (offset < end) {
 		if (!sectors_read) {
-reread:			sectors_read = min_t(unsigned,
+			struct bio *bio;
+			unsigned nr_bvecs;
+reread:
+			sectors_read = min_t(unsigned,
 				end - offset, buf->size >> 9);
+			nr_bvecs = buf_pages(buf->data, sectors_read << 9);
+
+			bio = bio_kmalloc(nr_bvecs, GFP_KERNEL);
+			bio_init(bio, ca->disk_sb.bdev, bio->bi_inline_vecs, nr_bvecs, REQ_OP_READ);
 
-			bio_reset(bio, ca->disk_sb.bdev, REQ_OP_READ);
 			bio->bi_iter.bi_sector	= offset;
 			bio->bi_iter.bi_size	= sectors_read << 9;
 			bch2_bio_map(bio, buf->data);
 
 			ret = submit_bio_wait(bio);
+			kfree(bio);
 
 			if (bch2_dev_io_err_on(ret, ca,
 					       "journal read from sector %llu",