tile: fix tilegx vmalloc_sync_all BUG_ON

As specified, the test wasn't correct, and in any case it should
be a BUILD_BUG_ON.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
diff --git a/arch/tile/mm/fault.c b/arch/tile/mm/fault.c
index f7f99f9..6152819 100644
--- a/arch/tile/mm/fault.c
+++ b/arch/tile/mm/fault.c
@@ -870,7 +870,8 @@
 {
 #ifdef __tilegx__
 	/* Currently all L1 kernel pmd's are static and shared. */
-	BUG_ON(pgd_index(VMALLOC_END) != pgd_index(VMALLOC_START));
+	BUILD_BUG_ON(pgd_index(VMALLOC_END - PAGE_SIZE) !=
+		     pgd_index(VMALLOC_START));
 #else
 	/*
 	 * Note that races in the updates of insync and start aren't