net/mlx5: fix Rx scatter mode validation

net/mlx5: fix Rx scatter mode validation

If the mbuf size of the Rx mempool supplied by the user in the Rx setup
is unable to contain the maximum Rx packet length in addition to the
mbuf head-room, the Rx scatter offload must be configured. Otherwise,
there is not enough space in single mbuf to contain a packet with size
of the maximum Rx packet length.

The PMD did not return an error in the above mentioned case.

Return an error in the above case.

Fixes: 7d6bf6b866b8 ("net/mlx5: add Multi-Packet Rx support")
Fixes: edad38fcd00e ("net/mlx: enhance Rx scatter mode detection")
Cc: stable@dpdk.org
Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>