selftests: ublk: common: fix _get_disk_dev_t for pre-9.0 coreutils

selftests: ublk: common: fix _get_disk_dev_t for pre-9.0 coreutils

Some distributions, such as centos stream 9, still have a version of
coreutils which does not yet support the %Hr and %Lr formats for stat(1)
[1, 2]. Running ublk selftests on these distributions results in the
following error in tests that use the _get_disk_dev_t helper:

line 23: ?r: syntax error: operand expected (error token is "?r")

To better accommodate older distributions, rewrite _get_disk_dev_t to
use the much older %t and %T formats for stat instead.

[1] https://github.com/coreutils/coreutils/blob/v9.0/NEWS#L114
[2] https://pkgs.org/download/coreutils

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250423-ublk_selftests-v1-2-7d060e260e76@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>