Handle the case when GSO SKB linear length is too large.

MANA NIC requires GSO packets to put only the header part to SGE0,
otherwise the TX queue may stop at the HW level.

So, use 2 SGEs for the skb linear part which contains more than the
packet header.

Fixes: ca9c54d2 ("net: mana: Add a driver for Microsoft Azure Network Adapter (MANA)")
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>