selftests/ublk: add UBLK_F_SHMEM_ZC support for loop target

selftests/ublk: add UBLK_F_SHMEM_ZC support for loop target

Add loop_queue_shmem_zc_io() which handles I/O requests marked with
UBLK_IO_F_SHMEM_ZC. When the kernel sets this flag, the request data
lives in a registered shared memory buffer — decode index + offset
from iod->addr and use the server's mmap as the I/O buffer.

The dispatch check in loop_queue_tgt_rw_io() routes SHMEM_ZC requests
to this new function, bypassing the normal buffer registration path.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-7-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>