s390: make extables read-only

Currently, exception tables are marked as ro_after_init. However,
since they are sorted during compile time using scripts/sorttable,
they can be moved to RO_DATA using the RO_EXCEPTION_TABLE_ALIGN macro,
which is specifically designed for this purpose.
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/vmlinux.lds.S

@@ -14,6 +14,8 @@
 #define BSS_FIRST_SECTIONS *(.bss..swapper_pg_dir) \
 			   *(.bss..invalid_pg_dir)
 
+#define RO_EXCEPTION_TABLE_ALIGN	16
+
 /* Handle ro_after_init data on our own. */
 #define RO_AFTER_INIT_DATA
 
@@ -66,7 +68,6 @@ SECTIONS
 		 *(.data..ro_after_init)
 		JUMP_TABLE_DATA
 	} :data
-	EXCEPTION_TABLE(16)
 	. = ALIGN(PAGE_SIZE);
 	__end_ro_after_init = .;