checkpatch: improve "no space after cast" test

Some false positives exist on this test.

For instance:
	*va_arg(args, signed char *) = val.s;
or
	memset(foo, 0, sizeof(struct bar *) * baz));

Ignore lines that have an arithmetic operator or assignment
after what appears to be a cast to a pointer "(foo *)".

Add $Arithmetic convenience variable.

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