selftests/ublk: add shared memory zero-copy support in kublk

selftests/ublk: add shared memory zero-copy support in kublk

Add infrastructure for UBLK_F_SHMEM_ZC shared memory zero-copy:

- kublk.h: struct ublk_shmem_entry and table for tracking registered
  shared memory buffers
- kublk.c: per-device unix socket listener that accepts memfd
  registrations from clients via SCM_RIGHTS fd passing. The listener
  mmaps the memfd and registers the VA range with the kernel for PFN
  matching. Also adds --shmem_zc command line option.
- kublk.c: --htlb <path> option to open a pre-allocated hugetlbfs
  file, mmap it with MAP_SHARED|MAP_POPULATE, and register it with
  the kernel via ublk_ctrl_reg_buf(). Any process that mmaps the same
  hugetlbfs file shares the same physical pages, enabling zero-copy
  without socket-based fd passing.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-6-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>