netfilter: xt_LOG: don't use xchg() for simple assignment

At least on ia64 the (bogus) use of xchg() here results in the compiler
warning about an unused expression result. As only an assignment is
intended here, convert it to such.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>