checkpatch: Warn on improper usage of Co-developed-by

The purpose of Co-developed-by: is to give attribution to authors who
aren't already attributed by the From: tag, i.e. who aren't the nominal
patch author.  Because Co-developed-by: is essentially a variation of
From:, it must be accompanied by a Signed-off-by: of the associated
co-author.  To ease the burden of determining whether or not co-authors
have signed off, Co-developed-by and Signed-off-by: must be explicitly
paired, i.e. on consecutive lines for a given co-author.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>