kunit: Add kunit_add_action() to defer a call until test exit

Many uses of the KUnit resource system are intended to simply defer
calling a function until the test exits (be it due to success or
failure). The existing kunit_alloc_resource() function is often used for
this, but was awkward to use (requiring passing NULL init functions, etc),
and returned a resource without incrementing its reference count, which
-- while okay for this use-case -- could cause problems in others.

Instead, introduce a simple kunit_add_action() API: a simple function
(returning nothing, accepting a single void* argument) can be scheduled
to be called when the test exits. Deferred actions are called in the
opposite order to that which they were registered.

This mimics the devres API, devm_add_action(), and also provides
kunit_remove_action(), to cancel a deferred action, and
kunit_release_action() to trigger one early.

This is implemented as a resource under the hood, so the ordering
between resource cleanup and deferred functions is maintained.

Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>