kunit: generalize kunit_resource API beyond allocated resources

kunit: generalize kunit_resource API beyond allocated resources

In its original form, the kunit resources API - consisting the
struct kunit_resource and associated functions - was focused on
adding allocated resources during test operation that would be
automatically cleaned up on test completion.

The recent RFC patch proposing converting KASAN tests to KUnit [1]
showed another potential model - where outside of test context,
but with a pointer to the test state, we wish to access/update
test-related data, but expressly want to avoid allocations.

It turns out we can generalize the kunit_resource to support
static resources where the struct kunit_resource * is passed
in and initialized for us. As part of this work, we also
change the "allocation" field to the more general "data" name,
as instead of associating an allocation, we can associate a
pointer to static data.  Static data is distinguished by a NULL
free functions.  A test is added to cover using kunit_add_resource()
with a static resource and data.

Finally we also make use of the kernel's krefcount interfaces
to manage reference counting of KUnit resources.  The motivation
for this is simple; if we have kernel threads accessing and
using resources (say via kunit_find_resource()) we need to
ensure we do not remove said resources (or indeed free them
if they were dynamically allocated) until the reference count
reaches zero.  A new function - kunit_put_resource() - is
added to handle this, and it should be called after a
thread using kunit_find_resource() is finished with the
retrieved resource.

We ensure that the functions needed to look up, use and
drop reference count are "static inline"-defined so that
they can be used by builtin code as well as modules in
the case that KUnit is built as a module.

A cosmetic change here also; I've tried moving to
kunit_[action]_resource() as the format of function names
for consistency and readability.

[1] https://lkml.org/lkml/2020/2/26/1286

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>