net/mlx5e: Fix UDP GSO for encapsulated packets

When the skb is encapsulated, adjust the inner UDP header instead of the
outer one, and account for UDP header (instead of TCP) in the inline
header size calculation.

Fixes: 689adf0d

 ("net/mlx5e: Add UDP GSO support")
Reported-by: Jason Baron <jbaron@akamai.com>
Closes: https://lore.kernel.org/netdev/c42961cb-50b9-4a9a-bd43-87fe48d88d29@akamai.com/
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Boris Pismenny <borisp@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>