dpif-linux: Avoid segfault on netdev_get_stats() without kernel module.

dpif-linux: Avoid segfault on netdev_get_stats() without kernel module.

netdev_linux_get_stats() calls into netdev_vport_get_stats(), which in
turn attempts a transaction on genl_sock.  If the kernel module isn't
loaded, then genl_sock won't be there, and in any case there's nothing that
guarantees that it's been initialized yet.

This fixes the problem by ensuring that dpif_linux was initialized properly
before attempting a transaction on genl_sock.

Reported-by: Aaron Rosen <arosen@clemson.edu>