Allow more flexibility in zone start and span

Allow more flexibility in zone start and span

Allow sequential read to start anywhere in a zone (option
offset), and have a span smaller than a zone (option size).

A use case is a Key Value Store reading a set of keys or values
starting somewhere in a zone.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Pierre Labat <plabat@micron.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>