asm-generic/atomic.h: simplify inc/dec test helpers

We already declared inc/dec helpers, so we don't need to call the
atomic_{add,sub}_return funcs directly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Arun Sharma <asharma@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>