ipvlan: NULL pointer dereference panic in ipvlan_port_destroy

When call to register_netdevice() (called from ipvlan_link_new()) fails,
we call ipvlan_uninit() (through ndo_uninit()) to destroy the ipvlan
port. After returning unsuccessfully from register_netdevice() we go
ahead and call ipvlan_port_destroy() again which causes NULL pointer
dereference panic. Fix the issue by making ipvlan_init() and
ipvlan_uninit() call symmetric.

The ipvlan port will now be created inside ipvlan_init() and will be
destroyed in ipvlan_uninit().

Fixes: 2ad7bf36 (ipvlan: Initial check-in of the IPVLAN driver)
Signed-off-by: Girish Moodalbail <girish.moodalbail@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>