datapath: Correct version check for genetlink net namespaces.

datapath: Correct version check for genetlink net namespaces.

Network namespaces were introduced in 2.6.24, not 2.6.19 as some of the
version checks were looking for.  This caused compile errors in that
kernel range when the functions couldn't be found.  In addition,
namespace support wasn't initially available in Netlink so the netns
headers weren't included, causing compile problems for a few more
kernels.

Reported-by: Justin Pettit <jpettit@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>