io_uring/zcrx: prepare fallback for larger pages

io_uring/zcrx: prepare fallback for larger pages

io_zcrx_copy_chunk() processes one page at a time, which won't be
sufficient when the net_iov size grows. Introduce a structure keeping
the target niov page and other parameters, it's more convenient and can
be reused later. And add a helper function that can efficient copy
buffers of an arbitrary length. For 64bit archs the loop inside should
be compiled out.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e84bc705a4e1edeb9aefff470d96558d8232388f.1751466461.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>