ipvlan: fix sparse warnings

Fix sparse warnings reported by kbuild robot

drivers/net/ipvlan/ipvlan_main.c:172:13: warning: symbol 'ipvlan_start_xmit' was not declared. Should it be static?
drivers/net/ipvlan/ipvlan_main.c:256:33: warning: incorrect type in initializer (different address spaces)
drivers/net/ipvlan/ipvlan_main.c:256:33:    expected void const [noderef] <asn:3>*__vpp_verify
drivers/net/ipvlan/ipvlan_main.c:256:33:    got struct ipvl_pcpu_stats *<noident>
drivers/net/ipvlan/ipvlan_main.c:544:5: warning: symbol 'ipvlan_link_register' was not declared. Should it be static

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>