btrfs: check-integrity: use proper helper to access btrfs_header

There's one raw use of le->cpu conversion but we have a helper to do
that for us, so use it.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/check-integrity.c

@@ -954,7 +954,7 @@ static noinline_for_stack int btrfsic_process_metablock(
 	sf->prev = NULL;
 
 continue_with_new_stack_frame:
-	sf->block->generation = le64_to_cpu(sf->hdr->generation);
+	sf->block->generation = btrfs_stack_header_generation(sf->hdr);
 	if (0 == sf->hdr->level) {
 		struct btrfs_leaf *const leafhdr =
 		    (struct btrfs_leaf *)sf->hdr;