net: vlan: add support for tunnel offload

GSO tunneled packets are always segmented in software before they are
transmitted by a VLAN, even when the lower device can offload tunnel
encapsulation and VLAN together (i.e., some bits in NETIF_F_GSO_ENCAP_ALL
mask are set in the lower device 'vlan_features'). If we let VLANs have
the same tunnel offload capabilities as their lower device, throughput
can improve significantly when CPU is limited on the transmitter side.

 - set NETIF_F_GSO_ENCAP_ALL bits in the VLAN 'hw_features', to ensure
 that 'features' will have those bits zeroed only when the lower device
 has no hardware support for tunnel encapsulation.
 - for the same reason, copy GSO-related bits of 'hw_enc_features' from
 lower device to VLAN, and ensure to update that value when the lower
 device changes its features.
 - set NETIF_F_HW_CSUM bit in the VLAN 'hw_enc_features' if 'real_dev'
 is able to compute checksums at least for a kind of packets, like done
 with commit 8403debe ("vlan: Keep NETIF_F_HW_CSUM similar to other
 software devices"). This avoids software segmentation due to mismatching
 checksum capabilities between VLAN's 'features' and 'hw_enc_features'.

Reported-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>