net/mlx5: zero LRO mbuf headroom

net/mlx5: zero LRO mbuf headroom

LRO packet may consume all the stride memory, hence the PMD cannot
guaranty head-room for the LRO mbuf.

The issue is lack in HW support to write the packet in offset from the
stride start.

A new striding RQ feature may be added in CX6 DX to allow head-room and
tail-room for the LRO strides.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>