kaiser: fix unlikely error in alloc_ldt_struct()

An error from kaiser_add_mapping() here is not at all likely, but
Eric Biggers rightly points out that __free_ldt_struct() relies on
new_ldt->size being initialized: move that up.
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

CVE-2017-5754
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/kernel/ldt.c

@@ -78,11 +78,11 @@ static struct ldt_struct *alloc_ldt_struct(int size)
 
 	ret = kaiser_add_mapping((unsigned long)new_ldt->entries, alloc_size,
 				 __PAGE_KERNEL);
+	new_ldt->size = size;
 	if (ret) {
 		__free_ldt_struct(new_ldt);
 		return NULL;
 	}
-	new_ldt->size = size;
 	return new_ldt;
 }