Fix panic triggered by code like this:

open("/dev/md0", O_EXEC);

Discussed with:	kib@, mav@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D3051

sys/geom/geom_dev.c

@@ -358,6 +358,13 @@ g_dev_open(struct cdev *dev, int flags, int fmt, struct thread *td)
 #else
 	e = 0;
 #endif
+
+	/*
+	 * This happens on attempt to open a device node with O_EXEC.
+	 */
+	if (r + w + e == 0)
+		return (EINVAL);
+
 	if (w) {
 		/*
 		 * When running in very secure mode, do not allow