vmlinux.lds: add missing VMLINUX_SYMBOL macros

When __{start,end}_ro_after_init is referenced from C code, we run into
the following build errors on blackfin:

  kernel/extable.c:169: undefined reference to `__start_ro_after_init'
  kernel/extable.c:169: undefined reference to `__end_ro_after_init'

The build error is due to the fact that blackfin is one of the few
arches that prepends an underscore '_' to all symbols defined in C.

Fix this by wrapping __{start,end}_ro_after_init in vmlinux.lds.h with
VMLINUX_SYMBOL(), which adds the necessary prefix for arches that have
HAVE_UNDERSCORE_SYMBOL_PREFIX.

Link: http://lkml.kernel.org/r/1491259387-15869-1-git-send-email-jeyu@redhat.com
Signed-off-by: Jessica Yu <jeyu@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Eddie Kovsky <ewk@edkovsky.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/vmlinux.lds.h

@@ -261,9 +261,9 @@
  */
 #ifndef RO_AFTER_INIT_DATA
 #define RO_AFTER_INIT_DATA						\
-	__start_ro_after_init = .;					\
+	VMLINUX_SYMBOL(__start_ro_after_init) = .;			\
 	*(.data..ro_after_init)						\
-	__end_ro_after_init = .;
+	VMLINUX_SYMBOL(__end_ro_after_init) = .;
 #endif
 
 /*