md: fix bug in raid10 recovery.

Adding a spare to a raid10 doesn't cause recovery to start.
This is due to an silly type in
  commit 6c2fce2e
and so is a bug in 2.6.27 and .28-rc.

Thanks to Thomas Backlund for bisecting to find this.

Cc: Thomas Backlund <tmb@mandriva.org>
Cc: stable@kernel.org

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