generic: mount overlayfs r/o if workdir cannot be created

generic: mount overlayfs r/o if workdir cannot be created

This is the case e.g. if JFFS2 partition is full.

see: https://www.mail-archive.com/linux-unionfs@vger.kernel.org/msg00246.html
(a small error in a logmessage was changed and reported upstream)

runtime tested on ar71xx with kernel 3.18.11 and r45772
this paritially fixes #19564

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>
Acked-by: Jo-Philipp Wich <jow@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45833 3c298f89-4303-0410-b956-a3cf2f4a3e73