selftests/ublk: add read-only buffer registration test

selftests/ublk: add read-only buffer registration test

Add --rdonly_shmem_buf option to kublk that registers shared memory
buffers with UBLK_SHMEM_BUF_READ_ONLY (read-only pinning without
FOLL_WRITE) and mmaps with PROT_READ only.

Add test_shmemzc_04.sh which exercises the new flag with a null target,
hugetlbfs buffer, and write workload. Write I/O works because the
server only reads from the shared buffer — the data flows from client
to kernel to the shared pages, and the server reads them out.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-11-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>