btrfs: make add_pending_csums() to take an ordered extent as parameter

btrfs: make add_pending_csums() to take an ordered extent as parameter

The structure btrfs_ordered_extent has a lot of list heads for different
purposes, passing a random list_head pointer is never a good idea as if
the wrong list is passed in, the type casting along with the fs will be
screwed up.

Instead pass the btrfs_ordered_extent pointer, and grab the csum_list
inside add_pending_csums() to make it a little safer.

Since we're here, also update the comments to follow the current style.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>