parisc: disable preemption during local tlb flush

flush_cache_mm() and flush_cache_range() fetch %sr3 via mfsp().
If it matches mm->context, they flush caches and the TLB. However,
the TLB is cpu-local, so if the code gets preempted shortly after
the mfsp(), and later resumed on another CPU, the wrong TLB is flushed.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>