[Yaffs] garbage collection control added to yaffs2

Hi All

I have just checked in a change which allows the control of garbage
collection.

This can be controlled via sysfs interface

At present there is only one bit being used but more bits will likely be added
in the future.

Disable gc
# echo 0 > /sys/module/yaffs/parameters/yaffs_gc_control

Enable gc
# echo 1 > /sys/module/yaffs/parameters/yaffs_gc_control

Disabling gc is generally not advised, but it can be used to write faster
during power down conditions etc.

-- Charles