checkpatch: avoid some commit message long line warnings

Fixes: and Link: lines may exceed 75 chars in the commit log.

So too can stack dump and dmesg lines and lines that seem
like filenames.

And Fixes: lines don't need to have a "commit" prefix before the
commit id.

Add exceptions for these types of lines.

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>