CRED: Wrap task credential accesses in the SunRPC protocol

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: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Neil Brown <neilb@suse.de>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: James Morris <jmorris@namei.org>

net/sunrpc/auth.c

@@ -351,8 +351,8 @@ struct rpc_cred *
 rpcauth_lookupcred(struct rpc_auth *auth, int flags)
 {
 	struct auth_cred acred = {
-		.uid = current->fsuid,
-		.gid = current->fsgid,
+		.uid = current_fsuid(),
+		.gid = current_fsgid(),
 		.group_info = current->group_info,
 	};
 	struct rpc_cred *ret;