netfilter: nfnetlink_log: fix wrong skbuff size calculation

This problem was introduced in 72961ecf


since no space was reserved for the new attributes NFULA_HWTYPE,
NFULA_HWLEN and NFULA_HWHEADER.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>