checkpatch: complex macros checks miss square brackets

We are missing 'simple' values which include square brackets.  Refactor to
ensure we handle nesting correctly and detect these simple forms.

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>