[PATCH] libata: clear ap->active_tag atomically w.r.t. command completion

ap->active_tag was cleared in ata_qc_free().  This left ap->active_tag
dangling after ata_qc_complete().  Spurious interrupts inbetween could
incorrectly access the qc.  Clear active_tag in ata_qc_complete().
This change is necessary for later EH changes.

Signed-off-by: Tejun Heo <htejun@gmail.com>