x86/alternatives: Sync core before enabling interrupts

text_poke_early() does:

   local_irq_save(flags);
   memcpy(addr, opcode, len);
   local_irq_restore(flags);
   sync_core();

That's not really correct because the synchronization should happen before
interrupts are reenabled to ensure that a pending interrupt observes the
complete update of the opcodes.

It's not entirely clear whether the interrupt entry provides enough
serialization already, but moving the sync_core() invocation into interrupt
disabled region does no harm and is obviously correct.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>