kunit: Use gfp in kunit_alloc_resource() kernel-doc

Copy/pasting the code from the kernel-doc here doesn't compile because
kunit_alloc_resource() takes a gfp flags argument. Pass the gfp
argument from the caller to complete the example.

Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>