[NETFILTER] nfnetlink: skip size check if size not specified (== 0)

Skip sizecheck if the size of the attribute wasn't specified, ie. zero.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>