netlink: Undo const marker in netlink_is_kernel().

We can't do this without propagating the const to nlk_sk()
too, otherwise:

net/netlink/af_netlink.c: In function ‘netlink_is_kernel’:
net/netlink/af_netlink.c:103:2: warning: passing argument 1 of ‘nlk_sk’ discards ‘const’ qualifier from pointer target type [enabled by default]
net/netlink/af_netlink.c:96:36: note: expected ‘struct sock *’ but argument is of type ‘const struct sock *’

Signed-off-by: David S. Miller <davem@davemloft.net>