block: Allow bio_iov_iter_get_pages() with bio->bi_bdev unset

bio_iov_iter_get_pages() trims the IO based on the block size of the
block device the IO will be issued to.

However, bcachefs is a multi device filesystem; when we're creating the
bio we don't yet know which block device the bio will be submitted to -
we have to handle the alignment checks elsewhere.

Thus this is needed to avoid a null ptr deref.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Link: https://lore.kernel.org/r/20230813182636.2966159-3-kent.overstreet@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Kent Overstreet

block/bio.c

@@ -1231,7 +1231,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	struct page **pages = (struct page **)bv;
 	ssize_t size, left;
 	unsigned len, i = 0;
-	size_t offset, trim;
+	size_t offset;
 	int ret = 0;
 
 	/*
@@ -1260,10 +1260,12 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 
 	nr_pages = DIV_ROUND_UP(offset + size, PAGE_SIZE);
 
-	trim = size & (bdev_logical_block_size(bio->bi_bdev) - 1);
-	iov_iter_revert(iter, trim);
+	if (bio->bi_bdev) {
+		size_t trim = size & (bdev_logical_block_size(bio->bi_bdev) - 1);
+		iov_iter_revert(iter, trim);
+		size -= trim;
+	}
 
-	size -= trim;
 	if (unlikely(!size)) {
 		ret = -EFAULT;
 		goto out;