clk: Drive clk_leaf_mux_set_rate_parent test from clk_ops

Running this kunit test with lockdep enabled leads to warning splats
about calling clk provider APIs without the clk_prepare lock held.  I
proposed adding a wrapper around these APIs to grab the prepare lock so
we can call them from anywhere, and Maxime implemented that approach[1],
but it didn't look great. That's because we had to make more kunit
testing APIs just to call code from a place that isn't a clk provider
when the prepare lock isn't held.

Instead of doing that, let's implement a determine_rate clk_op for a new
leaf clk that is the child of the existing leaf clk. We can call
__clk_determine_rate() on the existing leaf clk from there, and stash
away the clk_rate_request struct to check once the clk_op returns. Drive
that clk_op by calling clk_round_rate() to keep things similar to how it
was before (i.e. nothing actually changes rate, just the new rate is
determined). This silences the warning by driving the test from a
clk_op where we know the prepare lock is held.

While looking at this in more detail, it was determined that the code we
intended to test in commit 262ca38f

 ("clk: Stop forwarding
clk_rate_requests to the parent") wasn't actually tested. The call to
__clk_determine_rate() wasn't actually getting to the newly introduced
code under the CLK_SET_RATE_PARENT if condition in
clk_core_round_rate_nolock() because the parent clk (the mux) could
round rates. We introduce a new leaf and make sure the parent of that
clk has no clk_ops so that we can be certain that the
CLK_SET_RATE_PARENT condition in clk_core_round_rate_nolock() is
evaluated.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/linux-clk/2b594e50-2bbf-4a2d-88e6-49fc39f3957a@roeck-us.net/
Reported-by: kernel test robot <yujie.liu@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202301310919.b9d56ee3-yujie.liu@intel.com
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20230721-clk-fix-kunit-lockdep-v1-0-32cdba4c8fc1@kernel.org [1]
Fixes: 262ca38f

 ("clk: Stop forwarding clk_rate_requests to the parent")
Link: https://lore.kernel.org/r/20230912175534.2427862-2-sboyd@kernel.org
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>