net/mlx5e: When RXFCS is set, add FCS data into checksum calculation

[ Upstream commit 902a545904c71d719ed144234d67df75f31db63b ]

When RXFCS feature is enabled, the HW do not strip the FCS data,
however it is not present in the checksum calculated by the HW.

Fix that by manually calculating the FCS checksum and adding it to the SKB
checksum field.

Add helper function to find the FCS data for all SKB forms (linear,
one fragment or more).

Fixes: 102722fc ("net/mlx5e: Add support for RXFCS feature flag")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>