netlink: fix size comparison

[glebius]

Summary

We want to check the size of the header, not a pointer to it.

[markj]

sys/netlink/netlink_generic.c

70–71

It's nicer IMO to use sizeof(*hdr), although netlink is inconsistent about this.

[glebius]

sys/netlink/netlink_generic.c

70–71

I personally prefer sizeof(type) when coding protocols that exchange data, cause a protocol specification talks about types not about our local variable names. That makes it easier to check that code follow protocol spec. For other cases I prefer sizeof(*var), cause var type may change, and often does.