io_u: skip to the next zone when zoneskip is set to zero

io_u: skip to the next zone when zoneskip is set to zero

If zoneskip is not set or set to 0 in zonemode=strided mode, all IO
occurs in a single zone. If zoneskip is non-zero, the next zone begins
zoneskip bytes beyond the end of the current zone. Thus, it's not
possible to access contiguous zones since zoneskip bytes will separate
the end of one zone and the beginning of the next zone.

This patch allows fio to move to the next zone when zoneskip is
explicitly set to 0, making it possible to issue IO to contiguous zones.

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