x86_64: fix kernel rodata NX setting

Without CONFIG_DYNAMIC_FTRACE, mark_rodata_ro() would mark a wrong
number of pages as no-execute. The bug was introduced in the patch
"ftrace: dont write protect kernel text". The symptom was machine reboot
after a CPU hotplug.
Signed-off-by: Pekka Paalanen <pq@iki.fi>
Acked-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/mm/init_64.c

@@ -782,7 +782,7 @@ void mark_rodata_ro(void)
 	 * The rodata section (but not the kernel text!) should also be
 	 * not-executable.
 	 */
-	set_memory_nx(rodata_start, (end - start) >> PAGE_SHIFT);
+	set_memory_nx(rodata_start, (end - rodata_start) >> PAGE_SHIFT);
 
 	rodata_test();