datapath: Move Netlink PID for userspace actions from flows to actions.

datapath: Move Netlink PID for userspace actions from flows to actions.

Commit b063d9f06 "datapath: Use unicast Netlink sockets for upcalls" that
switched from multicast to unicast Netlink for sending upcalls added a
Netlink PID to each kernel flow, used by OVS_ACTION_ATTR_USERSPACE actions
within the flow as target.

This commit drops this per-flow PID in favor of a per-action PID, because
that is more flexible.  It does not yet make use of this additional
flexibility, so behavior should not change.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Bug #7559.