We very rarely (if ever) complete more than one command in the
sactive mask at the time, even for extremely high IO rates. So
looping over the entire range of possible tags is pointless,
instead use __ffs() to just find the completed tags directly.

Updated to clear the tag from the done_mask instead of shifting
done_mask down as suggested by From: Tejun Heo <htejun@gmail.com>
Verified with a user space tester to produce the same results.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>