header: make asm/atomic.h include asm/barrier.h

asm/barrier.h was created in kernel version 3.4, before that version
the code was in asm/atomic.h. Include it so we can add code to
asm/barrier.h and use it also in kernel < 3.4.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>