staging: lustre: ptlrpc: move thread creation out of module initialization

When the ptlrpc module is loaded, it starts the pinger thread and
calls LNetNIInit which starts various threads.

We don't need these threads until the module is actually being
used, such as when a lustre filesystem is mounted.

So move the thread creation into new ptlrpc_inc_ref() (modeled on
ptlrpcd_inc_ref()), and call that when needed, such as at mount time.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>