ublk: enable UBLK_F_SHMEM_ZC feature flag

ublk: enable UBLK_F_SHMEM_ZC feature flag

Add UBLK_F_SHMEM_ZC (1ULL << 19) to the UAPI header and UBLK_F_ALL.
Switch ublk_support_shmem_zc() and ublk_dev_support_shmem_zc() from
returning false to checking the actual flag, enabling the shared
memory zero-copy feature for devices that request it.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-4-ming.lei@redhat.com
[axboe: ublk_buf_reg -> ublk_shmem_buf_reg errors]
Signed-off-by: Jens Axboe <axboe@kernel.dk>