yaffs2: fix memory leak when /proc/yaffs is read

commit  27f18203551940abf35826a66978daf1b8124c6b from
git://www.aleph1.co.uk/yaffs2

Thanks to Jisheng Zhang <Jisheng.Zhang@synaptics.com> for supplying this patch

There is a kernel memory leak observed when the proc file /proc/yaffs
is read. This reason is that in yaffs_proc_open, single_open is called
and the respective release function is not called during release.

Fix with correct release function - single_release().

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>