cxgb4: Discard the packet if the length is greater than mtu

pktgen sends raw udp packets and bypasses most of the
linux networking stack. User can specify different packet sizes.
Hence we need to discard the packet if the length is greater than mtu

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>