selftests: ublk: add test for covering UBLK_AUTO_BUF_REG_FALLBACK

Add test for covering UBLK_AUTO_BUF_REG_FALLBACK:

- pass '--auto_zc_fallback' to null target, which requires both F_AUTO_BUF_REG
and F_SUPPORT_ZERO_COPY for handling UBLK_AUTO_BUF_REG_FALLBACK

- add ->buf_index() method for returning invalid buffer index to trigger
UBLK_AUTO_BUF_REG_FALLBACK

- add generic_09 for running the test

- add --auto_zc_fallback test in stress_03/stress_04/stress_05

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250520045455.515691-7-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>