config: Introduce RUN_ZONED_TESTS variable and CAN_BE_ZONED flag

config: Introduce RUN_ZONED_TESTS variable and CAN_BE_ZONED flag

To allow running tests using a null_blk device with the zoned mode
disabled (current setup) as well as enabled, introduce the config
the RUN_ZONED_TESTS config variable and the per-test flag CAN_BE_ZONED.

RUN_ZONED_TESTS=1 indicates that tests run against null_blk will be
executed twice, first with null_blk as a regular block device
(RUN_FOR_ZONED=0) and a second time with null_blk set as a zoned block
device (RUN_FOR_ZONED=1). This applies only to tests cases that have the
variable CAN_BE_ZONED set to 1, indicating that the test case applies to
zoned block devices. If CAN_BE_ZONED is not defined by a test case, the
test is executed only with the regular null_blk device.

_init_null_blk is modified to prepare null_blk as a zoned blocked device
if RUN_FOR_ZONED is set and as a regular block device otherwise. To avoid
"modprobe -r null_blk" failures, rmdir calls on all sysfs nullbX
directories is added.

When a zoned block device is specified in TEST_DEVS, failures of test
cases that do not set CAN_BE_ZONED are avoided by automatically skipping
the test. The new helper function _test_dev_is_zoned() is introduced to
implement this.

The use of the RUN_ZONED_TESTS variable requires that the kernel be
compiled with CONFIG_BLK_DEV_ZONED enabled.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>