checkpatch.pl: update from Linux kernel v4.16

Update scripts/checkpatch.pl from upstream.

One of the many corrections is not creating an error for cover-letters.

Reintroduce U-Boot's
5c761ce5
checkpatch.pl: Add warning for new __packed additions
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>