header: move smp_mb__after_atomic definition to the correct file

The smp_mb__after_atomic definition is in barrier.h in the mainline,
but we currently include the backport in atomic.h.  This may cause
problems with certain include chain combinations.

Move it to the correct place.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>