checkpatch: correctly track the end of preprocessor commands in context

When looking for a statement we currently run on through preprocessor
commands.  This means that a header file with just definitions is parsed
over and over again combining all of the lines from the current line to
the end of file leading to severe performance issues.

Fix up context accumulation to track preprocessor commands and stop when
reaching the end of them.  At the same time vastly simplify the #define
handling.

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