selftests/net: packetdrill: pass send_omit_free to MSG_ZEROCOPY tests

selftests/net: packetdrill: pass send_omit_free to MSG_ZEROCOPY tests

The --send_omit_free flag is needed for TCP zero copy tests, to ensure
that packetdrill doesn't free the send() buffer after the send() call.

Fixes: 1e42f73fd3c2 ("selftests/net: packetdrill: import tcp/zerocopy")
Closes: https://lore.kernel.org/netdev/20251124071831.4cbbf412@kernel.org/
Suggested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251125234029.1320984-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>