datapath: Work around debugging for csum_and_copy_to_user().

Certain versions of XenServer add debugging to csum_and_copy_to_user()
in such a way that it changes the function signature.  This adds a
check and a workaround to allow us to build on these versions.

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