selftests: ublk: add stripe target

selftests: ublk: add stripe target

Add ublk stripe target which can take 1~4 underlying backing files
or block device, with stripe size 4k ~ 512K.

Add two basic tests(write verify & mkfs/mount/umount) over ublk/stripe.

This target is helpful to cover multiple IOs aiming at same
fixed/registered IO kernel buffer.

It is also capable of verifying vectored registered (kernel)buffers
in future for zero copy, so far it isn't supported yet.

Todo: support vectored registered kernel buffer for ublk/zc.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-9-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>