selftest: epoll_busy_poll: epoll busy poll tests

selftest: epoll_busy_poll: epoll busy poll tests

Add a simple test for the epoll busy poll ioctls, using the kernel
selftest harness.

This test ensures that the ioctls have the expected return codes and
that the kernel properly gets and sets epoll busy poll parameters.

The test can be expanded in the future to do real busy polling (provided
another machine to act as the client is available).

Signed-off-by: Joe Damato <jdamato@fastly.com>
Link: https://lore.kernel.org/r/20240508184008.48264-1-jdamato@fastly.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>