sfc: process RX event inner checksum flags

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>