On MAC check failure for readdir, use 'goto out' to use the common exit

handling, rather than returning directly to prevent leaking of vnode
reference/lock.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, NAI Labs

sys/compat/svr4/svr4_misc.c

@@ -316,7 +316,7 @@ again:
 #ifdef MAC
 	error = mac_check_vnode_readdir(td->td_ucred, vp);
 	if (error)
-		return (error);
+		goto out;
 #endif
 
 	error = VOP_READDIR(vp, &auio, fp->f_cred, &eofflag,