t/zbd: test that conventional zones are not locked during random i/o

t/zbd: test that conventional zones are not locked during random i/o

A recently fixed bug was caused by an unexpected conventional zone lock
during random I/O adjustment. Only sequential zones are supposed to be
locked, but the conventional zone lock was observed with a random
workload against an I/O region with mixed conventional and sequential
zones.

Add two test cases with the same workload to ensure that no similar
regression happens in the future. One case tests reads and the other
is for writes. As a related change, add the helper function
require_conv_zones() to check that the test target device has enough
conventional zones available.

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