x86/split_lock: Simplify reenabling

When split_lock_mitigate is disabled, each CPU needs its own delayed_work
structure. They are used to reenable split lock detection after its
disabling. But delayed_work structure must be correctly initialized after
its allocation.

Current implementation uses deferred initialization that makes the
split lock handler code unclear. The code can be simplified a bit
if the initialization is moved to the appropriate initcall.

sld_setup() is called before setup_per_cpu_areas(), thus it can't be used
for this purpose, so introduce an independent initcall for
the initialization.

[ mingo: Simplified the 'work' assignment line a bit more. ]

Signed-off-by: Maksim Davydov <davydov-max@yandex-team.ru>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250325085807.171885-1-davydov-max@yandex-team.ru