zbd: Improve read randomness

zbd: Improve read randomness

With zonemode=zbd, for random read operations with read_beyond_wp=0,
the zbd code will always adjust an I/O offset so that the I/O hits a
non empty zone. However, the adjustment always sets the I/O offset to
the start of the zone, resulting in a high device read cache hit rate
if the device has few zones written.

Improve randomness of read I/Os by adjusting the I/O offset to a random
value within the range of written data of the chosen zone. Also ensure
that the modified I/O does not cross over the zone wp position by
adjusting its size. Doing so, the search for a new zone is now done only
if the target zone cannot accomodate an I/O of at least min_bs bytes.

The comments have also been changed to better explain the code.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>