checkpatch: add an invalid patch separator test

checkpatch: add an invalid patch separator test

Some versions of tools that apply patches incorrectly allow lines that
start with 3 dashes and have additional content on the same line.

Checkpatch will now emit an ERROR on these lines and optionally convert
those lines from dashes to equals with --fix.

Link: https://lkml.kernel.org/r/6ec1ed08328340db42655287afd5fa4067316b11.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Ian Rogers <irogers@google.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Namhyung kim <namhyung@kernel.org>
Cc: Stehen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>