parisc: Protect huge page pte changes with spinlocks

PA-RISC doesn't have atomic instructions to modify page table entries, so it
takes spinlock in the TLB handler and modifies the page table entry
non-atomically. If you modify the page table entry without the spinlock, you
may race with TLB handler on another CPU and your modification may be lost.
Protect against that with usage of purge_tlb_start() and purge_tlb_end() which
handles the TLB spinlock.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v4.4