selftests/net: deflake GRO tests

selftests/net: deflake GRO tests

GRO tests are timing dependent and can easily flake. This is partially
mitigated in gro.sh by giving each subtest 3 chances to pass. However,
this still flakes on some machines. Reduce the flakiness by:

- Bumping retries to 6.
- Setting napi_defer_hard_irqs to 1 to reduce the chance that GRO is
  flushed prematurely. This also lets us reduce the gro_flush_timeout
  from 1ms to 100us.

Tested: Ran `gro.sh -t large` 1000 times. There were no failures with
this change. Ran inside strace to increase flakiness.

Signed-off-by: Kevin Krakauer <krakauer@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250226192725.621969-4-krakauer@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>