xfrm: Have af-specific init_tempsel() initialize family field of temporary selector

While adding MIGRATE support to strongSwan, Andreas Steffen noticed that
the selectors provided in XFRM_MSG_ACQUIRE have their family field
uninitialized (those in MIGRATE do have their family set).

Looking at the code, this is because the af-specific init_tempsel()
(called via afinfo->init_tempsel() in xfrm_init_tempsel()) do not set
the value.

Reported-by: Andreas Steffen <andreas.steffen@strongswan.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>