io_uring/zcrx: dmabuf backed zerocopy receive

io_uring/zcrx: dmabuf backed zerocopy receive

Add support for dmabuf backed zcrx areas. To use it, the user should
pass IORING_ZCRX_AREA_DMABUF in the struct io_uring_zcrx_area_reg flags
field and pass a dmabuf fd in the dmabuf_fd field.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20bb1890e60a82ec945ab36370d1fd54be414ab6.1746097431.git.asml.silence@gmail.com
Link: https://lore.kernel.org/io-uring/6e37db97303212bbd8955f9501cf99b579f8aece.1746547722.git.asml.silence@gmail.com
[axboe: fold in fixup]
Signed-off-by: Jens Axboe <axboe@kernel.dk>