checkpatch: add an error test for no space before comma

Using code like:

    int foo , bar;

is not preferred to:

    int foo, bar;

so emit an error on this style.

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>