selftests: ublk: add generic_06 for covering fault inject

Add one simple fault inject target, and verify if an application using ublk
device sees an I/O error quickly after the ublk server dies.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250416035444.99569-9-ming.lei@redhat.com


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