fs: yaffs2: get rid of write_super

Commit f0cd2dbb kills the VFS '->write_super' method along with
's_dirt' flag to avoid unnecessary wakeups every 5secs which
check for dirty super blocks to write out. The solution is to make
individual file system take care of writing dirty super blocks
by themselves. For YAFFS, schedule a delay job to do file system sync
only when the super blocks are touched.

Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>