x86/smp: Split sending INIT IPI out into a helper function

Putting CPUs into INIT is a safer place during kexec() to park CPUs.

Split the INIT assert/deassert sequence out so it can be reused.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Link: https://lore.kernel.org/r/20230615193330.551157083@linutronix.de