checkpatch: track #ifdef/#else/#endif when tracking blocks

When picking up a complete statement or block for analysis we cannot
simply track open/close/etc parenthesis we must take into account
preprocessor section boundaries.

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