kernel: export gluebi info to sysfs

Export the ubi_num and vol_id into the sysfs record of the gluebi-
emulated mtd device. Previously userspace didn't have a way to
map gluebi-emulated devices back to their corresponding ubi volumes.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40306 3c298f89-4303-0410-b956-a3cf2f4a3e73