btrfs: simplify tree block checksumming loop

static int csum_tree_block(struct extent_buffer *buf, u8 *result)

{

	struct btrfs_fs_info *fs_info = buf->fs_info;

	const int num_pages = fs_info->nodesize >> PAGE_SHIFT;

	SHASH_DESC_ON_STACK(shash, fs_info->csum_shash);

	unsigned long len;

	unsigned long cur_len;

	unsigned long offset = BTRFS_CSUM_SIZE;

	char *kaddr;

	unsigned long map_start;

	unsigned long map_len;

	int err;

	int i;

	shash->tfm = fs_info->csum_shash;

	crypto_shash_init(shash);

	kaddr = page_address(buf->pages[0]);

	crypto_shash_update(shash, kaddr + BTRFS_CSUM_SIZE,

			    PAGE_SIZE - BTRFS_CSUM_SIZE);

	len = buf->len - offset;

	while (len > 0) {

		/*

		 * Note: we don't need to check for the err == 1 case here, as

		 * with the given combination of 'start = BTRFS_CSUM_SIZE (32)'

		 * and 'min_len = 32' and the currently implemented mapping

		 * algorithm we cannot cross a page boundary.

		 */

		err = map_private_extent_buffer(buf, offset, 32,

					&kaddr, &map_start, &map_len);

		if (WARN_ON(err))

			return err;

		cur_len = min(len, map_len - (offset - map_start));

		crypto_shash_update(shash, kaddr + offset - map_start, cur_len);

		len -= cur_len;

		offset += cur_len;

	for (i = 1; i < num_pages; i++) {

		kaddr = page_address(buf->pages[i]);

		crypto_shash_update(shash, kaddr, PAGE_SIZE);

	}

	memset(result, 0, BTRFS_CSUM_SIZE);

	crypto_shash_final(shash, result);

	return 0;