datapath: compat: Introduce static key support

datapath: compat: Introduce static key support

Static keys allow the inclusion of seldom used features in
performance-sensitive fast-path kernel code, via a GCC feature and a
code patching technique. For more information:
    * https://www.kernel.org/doc/Documentation/static-keys.txt

Since upstream ovs kernel module now uses some static key API that was
introduced in v4.3 kernel, we shall backport them to the compat module
for older kernel supprots.

This backport is based on upstream net-next commit 11276d5306b8
("locking/static_keys: Add a new static_key interface").

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>