This change fixes the following compiler warning:

/usr/include/x86_64-linux-gnu/bits/error.h:40:5: warning: ‘gso_size’ may
be used uninitialized [-Wmaybe-uninitialized]
   40 |     __error_noreturn (__status, __errnum, __format,
   __va_arg_pack ());
         |
	 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	 udpgso_bench_rx.c: In function ‘main’:
	 udpgso_bench_rx.c:253:23: note: ‘gso_size’ was declared here
	   253 |         int ret, len, gso_size, budget = 256;

Fixes: 3327a9c4 ("selftests: add functionals test for UDP GRO")
Signed-off-by: Andrei Gherzan <andrei.gherzan@canonical.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20230201001612.515730-1-andrei.gherzan@canonical.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>