exec_elf: assert that the image vnode is still locked on return

Suggested and reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D33359

sys/kern/imgact_elf.c

@@ -1363,6 +1363,7 @@ __CONCAT(exec_, __elfN(imgact))(struct image_params *imgp)
 	imgp->proc->p_elf_flags = hdr->e_flags;
 
 ret:
+	ASSERT_VOP_LOCKED(imgp->vp, "skipped relock");
 	if (free_interp)
 		free(interp, M_TEMP);
 	return (error);