md: close some races between setting and checking sync_action.

When checking sync_action in a script, we want to be sure it is
as accurate as possible.
As resync/reshape etc doesn't always start immediately (a separate
thread is scheduled to do it), it is best if 'action_show'
checks if MD_RECOVER_NEEDED is set (which it does) and in that
case reports what is likely to start soon (which it only sometimes
does).

So:
 - report 'reshape' if reshape_position suggests one might start.
 - set MD_RECOVERY_RECOVER in raid1_reshape(), because that is very
   likely to happen next.

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