powerpc: Add function to copy mm_context_t to the paca

This adds a function to copy the mm->context to the paca.  This is
only a basic conversion for now but will be used more extensively in
the next patch.

This also adds #ifdef CONFIG_PPC_BOOK3S around this code since it's
not used elsewhere.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index 4233dcc..03279ea 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -882,7 +882,8 @@
 	slice_set_range_psize(mm, addr, 1, MMU_PAGE_4K);
 	copro_flush_all_slbs(mm);
 	if ((get_paca_psize(addr) != MMU_PAGE_4K) && (current->mm == mm)) {
-		get_paca()->context = mm->context;
+
+		copy_mm_to_paca(&mm->context);
 		slb_flush_and_rebolt();
 	}
 }
@@ -949,7 +950,7 @@
 {
 	if (user_region) {
 		if (psize != get_paca_psize(ea)) {
-			get_paca()->context = mm->context;
+			copy_mm_to_paca(&mm->context);
 			slb_flush_and_rebolt();
 		}
 	} else if (get_paca()->vmalloc_sllp !=