totemknet: Add locking for log call

Knet callbacks may be called from different thread than main thread. If
this happens, log messages may be lost. Most prominent example is when
link goes up (logged by main thread) and host_change_callback_fn is
called.

Implemented solution is adding mutex for every log call in totemknet.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>