selftests: fix poll error in udpgro.sh

The test program udpgso_bench_rx always invokes the poll()
syscall with a timeout of 10ms. If a larger timeout is specified
via the command line, udpgso_bench_rx is supposed to do multiple
poll() calls till the timeout is expired or an event is received.

Currently the poll() loop errors out after the first invocation with
no events, and may causes self-tests failure alike:

failed
 GRO with custom segment size            ./udpgso_bench_rx: poll: 0x0 expected 0x1

This change addresses the issue allowing the poll() loop to consume
all the configured timeout.

Fixes: ada641ff

 ("selftests: fixes for UDP GRO")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>