blkparse: Initialize and test for undefined request tracking timestamps

blkparse: Initialize and test for undefined request tracking timestamps

Currently, event tracking timestamps aren't initialized at all even though some
places in the code assume that a value of 0 indicates 'undefined'.  However, 0
is the timestamp of the first event, so use -1ULL for 'undefined' instead.

In addition, make sure timestamps are only initialized once, and always check
if timestamps are defined before using them.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>