Grow: split out start_reshape for initiating reshape via sysfs.

Grow: split out start_reshape for initiating reshape via sysfs.

Rather than sprinkling various sysfs setting around, put them all
in one place.  This will make implementing ->manage_reshape easier.

This changes behaviour slightly.
Previously we would not set 'sync_action' to 'reshape' until we were
ready for the process to start.  Now we set sync_max to zero and set
sync_action to 'reshape' at that time.  When we want reshape to
actually start we advance sync_max.

Signed-off-by: NeilBrown <neilb@suse.de>