tests: Avoid -Wcast-align warning

tests: Avoid -Wcast-align warning

warning: cast from 'struct sockaddr *' to 'struct sockaddr_in *'
                increases required alignment from 2 to 4 [-Wcast-align]
        struct sockaddr_in *sinp = (struct sockaddr_in *)to; ^~~~~~~~~~~~~~~~~~~~~~~~

Above warning can be avoided by making use of sockaddr_in available
within torture_address from the caller of echo_udp_recv_from_to().
Following that sa_socklen from torture_address can be used instead of
passing sa_socklen explicitly to echo_udp_recv_from_to().

Signed-off-by: Anoop C S <anoopcs@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>