parisc/mm: Ensure IRQs are off in switch_mm()

This is because of commit f98db601

 ("sched/core: Add switch_mm_irqs_off()
and use it in the scheduler") in which switch_mm_irqs_off() is called by the
scheduler, vs switch_mm() which is used by use_mm().

This patch lets the parisc code mirror the x86 and powerpc code, ie. it
disables interrupts in switch_mm(), and optimises the scheduler case by
defining switch_mm_irqs_off().

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v4.7+
Signed-off-by: Helge Deller <deller@gmx.de>