af_key: Fix selector family initialization.

This propagates the xfrm_user fix made in commit
bcf0dda8

 ("[XFRM]: xfrm_user: fix
selector family initialization")

Based upon a bug report from, and tested by, Alan Swanson.

Signed-off-by: Kazunori MIYAZAWA <kazunori@miyazawa.org>
Signed-off-by: David S. Miller <davem@davemloft.net>