In do_execve(), clear imgp->textset when restarting for interpreter.

Otherwise, we might left the boolean set, which would affect cleanup
after an error on interpreter activation.

Reviewed by:	markj
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D21560

sys/kern/kern_exec.c

@@ -616,7 +616,9 @@ do_execve(struct thread *td, struct image_args *args, struct mac *mac_p)
 		 * The vnode lock is held over this entire period
 		 * so nothing should illegitimately be blocked.
 		 */
-		VOP_UNSET_TEXT_CHECKED(imgp->vp);
+		MPASS(imgp->textset);
+		VOP_UNSET_TEXT_CHECKED(newtextvp);
+		imgp->textset = false;
 		/* free name buffer and old vnode */
 		if (args->fname != NULL)
 			NDFREE(&nd, NDF_ONLY_PNBUF);