checkpatch: update to linux-4.17

We also need to fix the update script to handle version output that
is sorted alphabetically rather than by versions.  Otherwise we keep
seeing 4.9 as the latest release instead of 4.1x.

Bug: chromium:850856
Test: `./checkpatch.pl-update` grabs 4.17
Change-Id: I821737ec685fda8d18b87edbf6112e3da54da67f