[NETFILTER]: fix sparse gfp nocast warnings

Fix implicit nocast warnings in nfnetlink code:
net/netfilter/nfnetlink.c:204:43: warning: implicit cast to nocast type

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>