userns: Allow PR_CAPBSET_DROP in a user namespace.

As the capabilites and capability bounding set are per user namespace
properties it is safe to allow changing them with just CAP_SETPCAP
permission in the user namespace.
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Tested-by: Richard Weinberger <richard@nod.at>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

security/commoncap.c

@@ -824,7 +824,7 @@ int cap_task_setnice(struct task_struct *p, int nice)
  */
 static long cap_prctl_drop(struct cred *new, unsigned long cap)
 {
-	if (!capable(CAP_SETPCAP))
+	if (!ns_capable(current_user_ns(), CAP_SETPCAP))
 		return -EPERM;
 	if (!cap_valid(cap))
 		return -EINVAL;