net/ncsi: use kfree_skb() instead of kfree()

We're supposed to use kfree_skb() to free these sk_buffs.

Fixes: 955dc68c ("net/ncsi: Add generic netlink family")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>