io_uring/zcrx: selftests: add test case for rss ctx

RSS contexts are used to shard work across multiple queues for an
application using io_uring zero copy receive. Add a test case checking
that steering flows into an RSS context works.

Until I add multi-thread support to the selftest binary, this test case
only has 1 queue in the RSS context.

Signed-off-by: David Wei <dw@davidwei.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250425022049.3474590-4-dw@davidwei.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>