ublk: update UBLK_F_SUPPORT_ZERO_COPY comment in UAPI header

ublk: update UBLK_F_SUPPORT_ZERO_COPY comment in UAPI header

UBLK_F_SUPPORT_ZERO_COPY has a very old comment describing the initial
idea for how zero-copy would be implemented. The actual implementation
added in commit 1f6540e2aabb ("ublk: zc register/unregister bvec") uses
io_uring registered buffers rather than shared memory mapping.
Remove the inaccurate remarks about mapping ublk request memory into the
ublk server's address space and requiring 4K block size. Replace them
with a description of the current zero-copy mechanism.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250621171015.354932-1-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>