checkpatch: handle comment/quote nesting correctly

Ensure that a close comment cannot incorrectly trigger in the middle of a
string.  Reported by Jaswinder Singh.

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