CRED: Wrap task credential accesses in the network device drivers

Wrap access to task credentials so that they can be separated more easily from
the task_struct during the introduction of COW creds.

Change most current->(|e|s|fs)[ug]id to current_(|e|s|fs)[ug]id().

Change some task->e?[ug]id to task_e?[ug]id().  In some places it makes more
sense to use RCU directly rather than a convenient wrapper; these will be
addressed by later patches.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: netdev@vger.kernel.org
Signed-off-by: James Morris <jmorris@namei.org>

drivers/net/tun.c

@@ -712,9 +712,9 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 
 		/* Check permissions */
 		if (((tun->owner != -1 &&
-		      current->euid != tun->owner) ||
+		      current_euid() != tun->owner) ||
 		     (tun->group != -1 &&
-		      current->egid != tun->group)) &&
+		      current_egid() != tun->group)) &&
 		     !capable(CAP_NET_ADMIN))
 			return -EPERM;
 	}