eth: fbnic: support TCP segmentation offload

eth: fbnic: support TCP segmentation offload

Add TSO support to the driver. Device can handle unencapsulated or
IPv6-in-IPv6 packets. Any other tunnel stacks are handled with
GSO partial.

Validate that the packet can be offloaded in ndo_features_check.
Main thing we need to check for is that the header geometry can
be expressed in the decriptor fields (offsets aren't too large).

Report number of TSO super-packets via the qstat API.

Link: https://patch.msgid.link/20250216174109.2808351-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>