A race exists where creating cgroups and also updating the priomap
may result in losing a priomap update. This is because priomap
writers are not protected by rtnl_lock.

Move priority writer into rtnl_lock()/rtnl_unlock().

CC: Neil Horman <nhorman@tuxdriver.com>
Reported-by: NAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: NJohn Fastabend <john.r.fastabend@intel.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>