Currently copy between io request buffer(pages) and userspace buffer is
done inside ublk_map_io() or ublk_unmap_io(). This way performs very
well in case of pre-allocated userspace io buffer.

For dynamically allocated or external userspace backend io buffer,
UBLK_F_NEED_GET_DATA is added for ublk server to provide buffer by one
extra command communication for WRITE request. For READ, userspace
simply provides buffer, but can't know when the buffer is done[1].

Add UBLK_F_USER_COPY by moving io data copy out of kernel by providing
read()/write() on /dev/ublkcN, and simply let ublk server do the io
data copy. This way makes both side cleaner, the cost is that one extra
syscall for copy io data between request and backend buffer.

With UBLK_F_USER_COPY, it actually becomes possible to run per-io zero
copy now, such as, only do zero copy for big size IO, so it can be
thought as one prep patch for supporting zero copy. Meantime zero copy
still needs to expose read()/write() buffer for some corner case, such
as passthrough IO.

[1] READ buffer in UBLK_F_NEED_GET_DATA
https://lore.kernel.org/linux-block/116d8a56-0881-56d3-9bcc-78ff3e1dc4e5@linux.alibaba.com/T/#m23bd4b8634c0a054e6797063167b469949a247bb

ublksrv loop usercopy code:

	https://github.com/ming1/ubdsrv/commits/usercopySigned-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230519065030.351216-8-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>