datapath: Add version info for out-of-tree modules.

datapath: Add version info for out-of-tree modules.

The upstream version of the module always has the version of the running kernel
but for out-of-tree modules it can be difficult to tell the current version.
This adds the information to the module where it can be read using modinfo for
the on-disk version or from /sys/module/openvswitch/version for the currently
loaded module.

Suggested-by: Duffie Cooley <dcooley@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>