Revision 220937 in upstream removed a similar kludge, which had become
unnecessary in some cases but not in the one that concerns us (see
comment below).

--- a/sys/kern/vfs_mount.c
+++ b/sys/kern/vfs_mount.c
@@ -639,6 +639,24 @@
 	}
 
 	/*
+	 * If MNT_RDONLY was not specified as a mount option, and we
+	 * are trying to update a mount-point from "ro", it means
+	 * read-write mode is desired by userland even if "rw" or
+	 * "noro" weren't specified explicitly. Add "noro" here to
+	 * make that happen.
+	 */
+	if (!(fsflags & MNT_RDONLY)) {
+		struct vfsopt *noro_opt;
+		noro_opt = malloc(sizeof(struct vfsopt), M_MOUNT, M_WAITOK);
+		noro_opt->name = strdup("noro", M_MOUNT);
+		noro_opt->value = NULL;
+		noro_opt->len = 0;
+		noro_opt->pos = -1;
+		noro_opt->seen = 1;
+		TAILQ_INSERT_TAIL(optlist, noro_opt, link);
+	}
+
+	/*
 	 * Be ultra-paranoid about making sure the type and fspath
 	 * variables will fit in our mp buffers, including the
 	 * terminating NUL.