Add support for RX checksum offload of encapsulated packets. This
 essentially just means paying attention to the inner checksum flags
 in the RX event, and if *either* checksum flag indicates a fail then
 don't tell the kernel that checksum offload was successful.
Also, count these checksum errors and export the counts to ethtool -S.

Test the most common "good" case of RX events with a single bitmask
 instead of a series of ifs.  Move the more specific error checking
 in to a separate function for clarity, and don't use unlikely() there
 since we know at least one of the bits is bad.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>