checkpatch: widen implied comment detection to allow multiple stars

Some people use double star '**' as a comment continuation, and start
comments with complete lines of stars.  Widen the implied comment
detection to pick these up.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>