[PARISC] Always spinlock tlb flush operations to ensure preempt safety

Since taking a spinlock disables preempt, and we need to spinlock tlb flush
on SMP for N class, we might as well just spinlock on uniprocessor machines
too.

Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>