net: ethtool: add missing string for NETIF_F_GSO_TUNNEL_REMCSUM

Commit e585f236 ("udp: Changes to udp_offload to support remote
checksum offload") added new GSO type and a corresponding netdev
feature, but missed Ethtool's 'netdev_features_strings' table.
Give it a name so it will be exposed to userspace and become available
for manual configuration.

v3:
 - decouple from "netdev_features_strings[] cleanup" series;
 - no functional changes.

v2:
 - don't split the "Fixes:" tag across lines;
 - no functional changes.

Fixes: e585f236

 ("udp: Changes to udp_offload to support remote checksum offload")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>