io_u: move to next zone even if zoneskip is unset

io_u: move to next zone even if zoneskip is unset

This patch makes fio's behavior under zonemode=strided conform to the
documentation:

    I/O happens in a single zone until zonesize bytes have been
    transferred. After that number of bytes has been transferred
    processing of the next zone starts.

With zonemode=strided, before commit 35f561eb, fio would only move to
the next zone when zoneskip > 0 and zonesize bytes were written. There
would always be zoneskip bytes between the end of one zone and the
beginning of the next zone. If zoneskip was not set or set to 0, all IO
would happen in the first zone.

Commit 35f561eb changed this so that fio would move to the next zone
upon writing zonesize bytes if zoneskip was explicitly set to a value >=
0. This option made it possible for zones to be contiguous. The
documentation was not updated to reflect the new behavior.

I originally intended to submit a patch to update fio's documentation,
but upon further reflection it seems better to change fio's behavior and
have a clean user interface than to change the documentation to note
that zoneskip must be explciitly set in order for fio to move to the
next zone.

This patch also updates t/strided.py to reflect the new behavior.

Signed-off-by: Jens Axboe <axboe@kernel.dk>