selftests/net: call sendmmsg via udpgso_bench.sh

selftests/net: call sendmmsg via udpgso_bench.sh

Currently, sendmmsg is implemented in udpgso_bench_tx.c,
but it is not called by any test script.

This patch adds a test for sendmmsg in udpgso_bench.sh.
This allows for basic API testing and benchmarking
comparisons with GSO.

Signed-off-by: Kenjiro Nakayama <nakayamakenjiro@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20241203222843.26983-1-nakayamakenjiro@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>