net/mlx5e: Add UDP GSO support

This patch enables UDP GSO support. We enable this by using two WQEs
the first is a UDP LSO WQE for all segments with equal length, and the
second is for the last segment in case it has different length.
Due to HW limitation, before sending, we must adjust the packet length fields.

We measure performance between two Intel(R) Xeon(R) CPU E5-2643 v2 @3.50GHz
machines connected back-to-back with Connectx4-Lx (40Gbps) NICs.
We compare single stream UDP, UDP GSO and UDP GSO with offload.
Performance:
		| MSS (bytes)	| Throughput (Gbps)	| CPU utilization (%)
UDP GSO offload	| 1472		| 35.6			| 8%
UDP GSO 	| 1472		| 25.5			| 17%
UDP 		| 1472		| 10.2			| 17%
UDP GSO offload	| 1024		| 35.6			| 8%
UDP GSO		| 1024		| 19.2			| 17%
UDP 		| 1024		| 5.7			| 17%
UDP GSO offload	| 512		| 33.8			| 16%
UDP GSO		| 512		| 10.4			| 17%
UDP 		| 512		| 3.5			| 17%

Signed-off-by: Boris Pismenny <borisp@mellanox.com>
Signed-off-by: Yossi Kuperman <yossiku@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>