Add one-shot feature for yaffs auto checkpointing.

This feature is enabling by ORing in 0x04 to the yaffs_auto_checkpoint
module parameter. This will do a one-shot checkpoint on the next sync and
the bit will then be reset.

This is a nice way to force a checkpoint on the next sync. eg.
echo 6 > /sys/modules/yaffs/parameters/yaffs_auto_checkpoint
sync
yaffs_auto_checkpoint will now be 2

Signed-off-by: Charles Manning <cdhmanning@gmail.com>