powerpc/irq: Don't WARN continuously in arch_local_irq_restore()

When CONFIG_PPC_IRQ_SOFT_MASK_DEBUG is enabled (uncommon), we have a
series of WARN_ON's in arch_local_irq_restore().

These are "should never happen" conditions, but if they do happen they
can flood the console and render the system unusable. So switch them
to WARN_ON_ONCE().

Fixes: e2b36d59 ("powerpc/64: Don't trace code that runs with the soft irq mask unreconciled")
Fixes: 9b81c021 ("powerpc/64s: make PACA_IRQ_HARD_DIS track MSR[EE] closely")
Fixes: 7c0482e3 ("powerpc/irq: Fix another case of lazy IRQ state getting out of sync")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190708061046.7075-1-mpe@ellerman.id.au