selftests: ioam: refactoring to align with the fix

ioam6_parser uses a packet socket. After the fix to prevent writing to
cloned skb's, the receiver does not see its IOAM data anymore, which
makes input/forward ioam-selftests to fail. As a workaround,
ioam6_parser now uses an IPv6 raw socket and leverages ancillary data to
get hop-by-hop options. As a consequence, the hook is "after" the IOAM
data insertion by the receiver and all tests are working again.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>