checkpatch: ensure we only apply checks to the lines within hunks

We should only apply source checks to lines within hunks.  Checks which
are anchored in the context may falsly trigger in the commentory.  Ensure
they only match within valid hunk lines.

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>