Revert "userns: prevent speculative execution"

CVE-2017-5753 (revert embargoed)
CVE-2017-5715 (revert embargoed)

This reverts commit 9c852c53.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

kernel/user_namespace.c

@@ -502,10 +502,8 @@ static void *m_start(struct seq_file *seq, loff_t *ppos,
 	struct uid_gid_extent *extent = NULL;
 	loff_t pos = *ppos;
 
-	if (pos < map->nr_extents) {
-		gmb();
+	if (pos < map->nr_extents)
 		extent = &map->extent[pos];
-	}
 
 	return extent;
 }