datapath: Correctly update IP checksum with actions.

datapath: Correctly update IP checksum with actions.

The update_csum() function that we currently use to update
checksums on actions is really intended for L4 checksums.  In
particular, if the packet has a partial checksum and the field
is not in the pseudo header, it doesn't do anything at all.
This doesn't make sense for the IP header because Linux doesn't
use hardware offload for it, so we always need to recompute the
checksum.  Instead, we can use the kernel function csum_replace4(),
which will always do the right thing.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>