ethdev: fix data room size verification in Rx queue setup

ethdev: fix data room size verification in Rx queue setup

In the rte_eth_rx_queue_setup API function, the local variable named
mbp_buf_size, which is the data room size of the input parameter mp,
is checked to guarantee that each memory chunk used for net device
in the mbuf is bigger than the min_rx_bufsize. But if mbp_buf_size is
less than RTE_PKTMBUF_HEADROOM, the value of the following  statement
will be a large number since the mbp_buf_size is a unsigned value.
    mbp_buf_size - RTE_PKTMBUF_HEADROOM
As a result, it will cause a segment fault in this situation.

This patch fixes it by modify the check condition to guarantee that the
local variable named mbp_buf_size is bigger than RTE_PKTMBUF_HEADROOM.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Sachin Saxena <sachin.saxena@nxp.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>