checkpatch: macros: fix statement counting block end detection

We are incorrectly counting the lines in a block while accumulating
the trailing lines in a macro statement, leading to false positives.
Fix end of block handling and general counting for negative context 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>