t/zbd: Reset all zones before test when max open zones is specified

t/zbd: Reset all zones before test when max open zones is specified

When the test target device has maximum open zones limit, the zones in
test target region may not be opened up to the limit, because the zones
out of the test target region may have open zones. To ensure that the
test target zones can be opened up to the limit, reset all zones of the
test target device before the test cases with write work load starts.
Introduce the helper function prep_write() to check if all zone reset is
required and do the reset.

Also remove unnecessary reset_zone calls for test case #29 and #48. These
are no longer required by virtue of the improvement in zbd_setup_files()
to set up zones to meet max_open_zones limit.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>