[NETFILTER] nfnetlink: nfattr_parse() can never fail, make it void

nfattr_parse (and thus nfattr_parse_nested) always returns success. So we
can make them 'void' and remove all the checking at the caller side.

Based on original patch by Pablo Neira Ayuso <pablo@netfilter.org>

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