datapath: Reformat copyright messages.

datapath: Reformat copyright messages.

Many of our kernel copyright messages make reference to code being
copied from the Linux kernel, which is a bit odd for code in the
kernel.  This changes them to use the standard GNU GPL boilerplate
instead.  It does not change the actual license, which continues to
be GPLv2.

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