Block creation of the new nodes for read-only tmpfs mounts.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D19737

sys/fs/tmpfs/tmpfs_subr.c

@@ -218,6 +218,8 @@ tmpfs_alloc_node(struct mount *mp, struct tmpfs_mount *tmp, enum vtype type,
 		 */
 		return (EBUSY);
 	}
+	if ((mp->mnt_kern_flag & MNT_RDONLY) != 0)
+		return (EROFS);
 
 	nnode = (struct tmpfs_node *)uma_zalloc_arg(tmp->tm_node_pool, tmp,
 	    M_WAITOK);