ipv4: Delete redundant sk_family assignment

sk_alloc now sets sk_family so this is redundant.  In fact it caught
my eye because sock_init_data already uses sk_family so this is too
late anyway.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>