[AF_KEY]: fix sparse gfp nocast warnings

Fix implicit nocast warnings in net/key code:
net/key/af_key.c:195:27: warning: implicit cast to nocast type
net/key/af_key.c:1439:28: warning: implicit cast to nocast type

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