Unbreak MNT_UPDATE when running with cd as root. Detect mountroot by

checking for "path == NULL" (like ffs) rather than MNT_ROOT.  Otherwise
when you try and do an update or mountd does an NFS export, the remount
fails because the code tries to mount a fresh rootfs and gets an EBUSY.
The same bug is in 4.x (which is where I found it).

Sanity check by: mux

sys/fs/cd9660/cd9660_vfsops.c

@@ -196,9 +196,8 @@ cd9660_mount(mp, path, data, ndp, td)
 	mode_t accessmode;
 	struct iso_mnt *imp = 0;
 
-	if ((mp->mnt_flag & MNT_ROOTFS) != 0) {
+	if (path == NULL)	/* We are doing the initial root mount */
 		return (iso_mountroot(mp, td));
-	}
 	if ((error = copyin(data, (caddr_t)&args, sizeof (struct iso_args))))
 		return (error);
 

sys/isofs/cd9660/cd9660_vfsops.c

@@ -196,9 +196,8 @@ cd9660_mount(mp, path, data, ndp, td)
 	mode_t accessmode;
 	struct iso_mnt *imp = 0;
 
-	if ((mp->mnt_flag & MNT_ROOTFS) != 0) {
+	if (path == NULL)	/* We are doing the initial root mount */
 		return (iso_mountroot(mp, td));
-	}
 	if ((error = copyin(data, (caddr_t)&args, sizeof (struct iso_args))))
 		return (error);