md: simplify some 'if' conditionals in raid5_start_reshape.

There are two consecutive 'if' statements.

 if (mddev->delta_disks >= 0)
      ....
 if (mddev->delta_disks > 0)

The code in the second is equally valid if delta_disks == 0, and these
two statements are the only place that 'added_devices' is used.

So make them a single if statement, make added_devices a local
variable, and re-indent it all.

No functional change.

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