asm-generic: make bitops.h usable

bitops.h apparently suffered from some level of bitrot, it
was missing the smp_mb__{before,after}_clear_bit functions,
and included other headers in an invalid order.

This changes the file so that new architectures can use
it out of the box.

Signed-off-by: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>