selftests: ublk: fix UBLK_F_NEED_GET_DATA

Commit 57e13a2e ("selftests: ublk: support user recovery") starts to
support UBLK_F_NEED_GET_DATA for covering recovery feature, however the
ublk utility implementation isn't done correctly.

Fix it by supporting UBLK_F_NEED_GET_DATA correctly.

Also add test generic_07 for covering UBLK_F_NEED_GET_DATA.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 57e13a2e ("selftests: ublk: support user recovery")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250429022941.1718671-2-ming.lei@redhat.com


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