[XFRM]: xfrm_user: fix selector family initialization

Commit df9dcb45 ([IPSEC]: Fix inter address family IPsec tunnel handling)
broke openswan by removing the selector initialization for tunnel mode
in case it is uninitialized.

This patch restores the initialization, fixing openswan, but probably
breaking inter-family tunnels again (unknown since the patch author
disappeared). The correct thing for inter-family tunnels is probably
to simply initialize the selector family explicitly.

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