checkpatch: comment detection: ignore macro continuation when detecting associated comments

When looking for an associated comment they may be suffixed by a macro
continuation.  Ignore this.

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>