[POWERPC] Make high hugepage areas preempt safe

Checking source for other get_paca()->field preemption dangers found that
open_high_hpage_areas does a structure copy into its paca while preemption
is enabled: unsafe however gcc accomplishes it.  Just remove that copy:
it's done safely afterwards by on_each_cpu, as in open_low_hpage_areas.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/hugetlbpage.c

@@ -480,9 +480,6 @@ static int open_high_hpage_areas(struct mm_struct *mm, u16 newareas)
 
 	mm->context.high_htlb_areas |= newareas;
 
-	/* update the paca copy of the context struct */
-	get_paca()->context = mm->context;
-
 	/* the context change must make it to memory before the flush,
 	 * so that further SLB misses do the right thing. */
 	mb();