kunit: introduce kunit_kmalloc_array/kunit_kcalloc() helpers

Add in:
* kunit_kmalloc_array() and wire up kunit_kmalloc() to be a special
case of it.
* kunit_kcalloc() for symmetry with kunit_kzalloc()

This should using KUnit more natural by making it more similar to the
existing *alloc() APIs.

And while we shouldn't necessarily be writing unit tests where overflow
should be a concern, it can't hurt to be safe.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>