checkpatch: possible types -- prevent illegal modifiers being added

Prevent known non types being detected as modifiers.  Ensure we do not
look at any type which starts with a keyword.

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>