clk: avoid circular clock topology

Currently, clk_register() never checks a circular parent looping,
but clock providers could register such an insane clock topology.
For example, "clk_a" could have "clk_b" as a parent, and vice versa.
In this case, clk_core_reparent() creates a circular parent list
and __clk_recalc_accuracies() calls itself recursively forever.

The core infrastructure should be kind enough to bail out, showing
an appropriate error message in such a case.  This helps to easily
find a bug in clock providers.  (uh, I made such a silly mistake
when I was implementing my clock providers first.  I was upset
because the kernel did not respond, without any error message.)

This commit adds a new helper function, __clk_is_ancestor().  It
returns true if the second argument is a possible ancestor of the
first one.  If a clock core is a possible ancestor of itself, it
would make a loop when it were registered.  That should be detected
as an error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

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