io_uring/zcrx: don't set rx_page_size when not requested

The rx_buf_len parameter was recently added to the Rx zero-copy
implementation. The expectation is that when not set system will
maintain previous behavior and use the default buffer size (PAGE_SIZE).

This works correctly at the iouring level, but we don't preserve
the same "zero means default" semantics when registering the memory
provider on the netdev. mp_param.rx_page_size is unconditionally
set to PAGE_SIZE. This causes __net_mp_open_rxq() to check for
QCFG_RX_PAGE_SIZE support in the driver, and return -EOPNOTSUPP
for drivers that don't advertise it -- even though the user never
asked for large buffers.

Only set mp_param.rx_page_size when rx_buf_len was explicitly provided,
so that the default page size path works on all zcrx-capable drivers.
mlx5 and fbnic only support 4kB pages in the current release.

Fixes: 795663b4 ("io_uring/zcrx: implement large rx buffer support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>