blk-iolatency: truncate our current time

blk-iolatency: truncate our current time

In our longer tests we noticed that some boxes would degrade to the
point of uselessness.  This is because we truncate the current time when
saving it in our bio, but I was using the raw current time to subtract
from.  So once the box had been up a certain amount of time it would
appear as if our IO's were taking several years to complete.  Fix this
by truncating the current time so it matches the issue time.  Verified
this worked by running with this patch for a week on our test tier.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>