Merge branch 'selftests-net-deflake-gro-tests-and-fix-return-value-and-output'

Kevin Krakauer says:

====================
selftests/net: deflake GRO tests and fix return value and output

The GRO selftests can flake and have some confusing behavior. These
changes make the output and return value of GRO behave as expected, then
deflake the tests.

v1: https://lore.kernel.org/20250218164555.1955400-1-krakauer@google.com
====================

Link: https://patch.msgid.link/20250226192725.621969-1-krakauer@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>