powerpc/mm: Handle hypervisor pte insert failure in __hash_page_huge

If the hypervisor gives us an error on a hugepage insert we panic. The
normal page code already handles this by returning an error instead and we end
calling low_hash_fault which will just kill the task if possible.

The patch below does a similar thing for the hugepage case.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/hugetlbpage-hash64.c

@@ -121,8 +121,15 @@ repeat:
                         }
 		}
 
-		if (unlikely(slot == -2))
-			panic("hash_huge_page: pte_insert failed\n");
+		/*
+		 * Hypervisor failure. Restore old pte and return -1
+		 * similar to __hash_page_*
+		 */
+		if (unlikely(slot == -2)) {
+			*ptep = __pte(old_pte);
+			err = -1;
+			goto out;
+		}
 
 		new_pte |= (slot << 12) & (_PAGE_F_SECOND | _PAGE_F_GIX);
 	}