gre: minor cleanups in netlink interface

- use typeful helpers for IFLA_GRE_LOCAL/IFLA_GRE_REMOTE
- replace magic value by FIELD_SIZEOF
- use MODULE_ALIAS_RTNL_LINK macro

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>