multipathd: Fix uxlsnr race condition on shutdown

multipathd: Fix uxlsnr race condition on shutdown

The multipath daemon deallocates some memory structures
upon shutdown which have been allocated in the thread
context of uxlsnr. Upon shutdown this thread is already
done for, taking it's memory structures with it.
So we need to establish a proper pthread cleanup
handler here to ensure the memory structures are
freed correctly.

Signed-off-by: Hannes Reinecke <hare@suse.de>