io_uring/net: correct type for min_not_zero() cast

io_uring/net: correct type for min_not_zero() cast

The kernel test robot reports that after a recent change, the signedness
of a min_not_zero() compare is now incorrect. Fix that up and cast to
the right type.

Fixes: 429884ff35f7 ("io_uring/kbuf: use struct io_br_sel for multiple buffers picking")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509020426.WJtrdwOU-lkp@intel.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>