Without this initialization one gets

kernel BUG at kernel/rtmutex_common.h:80!

This patch should also be included in the -stable kernel.
Signed-off-by: G. Liakhovetski <gl@dsa-ac.de>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>