selftests/net: so_txtime: fix parsing of start time stamp on 32 bit systems

selftests/net: so_txtime: fix parsing of start time stamp on 32 bit systems

This patch fixes the parsing of the cmd line supplied start time on 32
bit systems. A "long" on 32 bit systems is only 32 bit wide and cannot
hold a timestamp in nano second resolution.

Fixes: 2c234ba69cdd ("selftests/net: so_txtime multi-host support")
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20220502094638.1921702-2-mkl@pengutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>