timers/migration: Fix hotplug migrator selection target on asymetric capacity machines

timers/migration: Fix hotplug migrator selection target on asymetric capacity machines

When a top-level migrator is deactivated, either at CPU down hotplug time
or when a CPU is domain isolated, a new migrator is elected among the
available CPUs and woken up to take over the migration duty.

However that election must happen at the scope of a given hierarchy and not
globally, which the introduction of per-capacity hierarchies failed to
handle.

As a result a given hierarchy may end up without migrator to handle global
timers.

Fix it by making sure that the new migrator belongs to the same hierarchy
as the outgoing CPU.

Fixes: 098cbaad8e57 ("timers/migration: Split per-capacity hierarchies")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260519220926.63437-2-frederic@kernel.org