md/raid10: fix refounct imbalance when resyncing an array with a replacement device.

If you have a raid10 with a replacement device that is resyncing -
e.g. after a crash before the replacement was complete - the write to
the replacement will increment nr_pending on the wrong device, which
will lead to strangeness.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>