netlink: add nl_set_extack_cookie_u64()

Add a helper function nl_set_extack_cookie_u64() to use a u64 as
the netlink extended ACK cookie, to avoid having to open-code it
in any users of the cookie.

A u64 should be sufficient for most subsystems though we allow
for up to 20 bytes right now. This also matches the cookies in
nl80211 where I intend to use this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>