/nfnetlink_acct.c: use linux/atomic.h

There's no known problem here, but this is one of only two non-arch files
in the kernel which use asm/atomic.h instead of linux/atomic.h.

Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>