datapath: Compute checksum while sending packets to userspace().

datapath: Compute checksum while sending packets to userspace().

Currently we compute the checksums on packets being sent to
userspace first and then copy them to a userspace buffer.  However,
these two operations can be combined for a significant savings
because the packet data only has to be loaded once.  This also
allows GSO packets to save an extra copy.

This will likely have an impact on NIC-121 because it eliminates
the code path that triggers the issue.  However, it is not a fix
for the root cause.