selftests: ublk: support UBLK_F_AUTO_BUF_REG

selftests: ublk: support UBLK_F_AUTO_BUF_REG

Enable UBLK_F_AUTO_BUF_REG support for ublk utility by argument `--auto_zc`,
meantime support this feature in null, loop and stripe target code.

Add function test generic_08 for covering basic UBLK_F_AUTO_BUF_REG feature.

Also cover UBLK_F_AUTO_BUF_REG in stress_03, stress_04 and stress_05 test too.

'fio/t/io_uring -p0 /dev/ublkb0' shows that F_AUTO_BUF_REG can improve
IOPS by 50% compared with F_SUPPORT_ZERO_COPY in my test VM.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250520045455.515691-6-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>