md/raid5: make sure curr_sync_completes is uptodate when reshape starts

This value is visible through sysfs and is used by mdadm
when it manages a reshape (backing up data that is about to be
rearranged).  So it is important that it is always correct.
Current it does not get updated properly when a reshape
starts which can cause problems when assembling an array
that is in the middle of being reshaped.

This is suitable for 2.6.31.y stable kernels.

Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>