net: rocker: set limit to DMA buffer size

Rocker network switch emulator has test registers to help debug
DMA operations. While testing host DMA access, a buffer address
is written to register 'TEST_DMA_ADDR' and its size is written to
register 'TEST_DMA_SIZE'. When performing TEST_DMA_CTRL_INVERT
test, if DMA buffer size was greater than 'INT_MAX', it leads to
an invalid buffer access. Limit the DMA buffer size to avoid it.

Reported-by: Huawei PSIRT <psirt@huawei.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
