livepatch: fix RCU usage in klp_find_external_symbol()

While one must hold RCU-sched (aka. preempt_disable) for find_symbol()
one must equally hold it over the use of the object returned.

The moment you release the RCU-sched read lock, the object can be dead
and gone.

[jkosina@suse.cz: change subject line to be aligned with other patches]
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Petr Mladek <pmladek@suse.cz>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/livepatch/core.c

@@ -248,11 +248,12 @@ static int klp_find_external_symbol(struct module *pmod, const char *name,
 	/* first, check if it's an exported symbol */
 	preempt_disable();
 	sym = find_symbol(name, NULL, NULL, true, true);
-	preempt_enable();
 	if (sym) {
 		*addr = sym->value;
+		preempt_enable();
 		return 0;
 	}
+	preempt_enable();
 
 	/* otherwise check if it's in another .o within the patch module */
 	return klp_find_object_symbol(pmod->name, name, addr);