io_uring/net: cast min_not_zero() type

io_uring/net: cast min_not_zero() type

kernel test robot reports that xtensa complains about different
signedness for a min_not_zero() comparison. Cast the int part to size_t
to avoid this issue.

Fixes: e227c8cdb47b ("io_uring/net: use passed in 'len' in io_recv_buf_select()")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507150504.zO5FsCPm-lkp@intel.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>