t/zbd: add checks for offline zone condition

t/zbd: add checks for offline zone condition

Some tests, e.g. #39 an #40, try to read the first zone of the drive.
It is assumed that the first zone is readable. However, if the first
zone is offline, the read fails along with the entire test.

This commit adds two functions to perform zone report and find the
first and the last zones that are not offline. Several test cases
now call these functions to avoid test failures described above.

Fixes for two more test failures are included in this commit -

Test #14 tries to write to conventional zones if they are found at
the beginning of the LBA range of the drive, but it assumes that
these zones are online. This may not always be the case. Add "offset"
to avoid the i/o to be attempted to run against any preceding offline
zones.

Similarly, in test #17, the script tries to find the last zone.
Check for the case when the last zone is offline. The test doesn't
set the i/o file size, but it works OK in most of the cases because
typically this test operates on the last physical zone. With the
online lookup in place, this may not always be the case and if there
are any offline zones that trail the last non-offline zone,
then the i/o will try to access that zone and fail. Add the "size"
to avoid the i/o to be attempted to run against any trailing offline
zones.

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