btrfs: rename btrfs_ordered_extent::list to csum_list

btrfs: rename btrfs_ordered_extent::list to csum_list

That list head records all pending checksums for that ordered
extent. And unlike other lists, we just use the name "list", which can
be very confusing for readers.

Rename it to "csum_list" which follows the remaining lists, showing the
purpose of the list.

And since we're here, remove a comment inside
btrfs_finish_ordered_zoned() where we have
"ASSERT(!list_empty(&ordered->csum_list))" to make sure the OE has
pending csums.

That comment is only here to make sure we do not call list_first_entry()
before checking BTRFS_ORDERED_PREALLOC.
But since we already have that bit checked and even have a dedicated
ASSERT(), there is no need for that comment anymore.

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