net, cgroup: cgroup_sk_updat_lock was missing initializer

bd1060a1 ("sock, cgroup: add sock->sk_cgroup") added global
spinlock cgroup_sk_update_lock but erroneously skipped initializer
leading to uninitialized spinlock warning.  Fix it by using
DEFINE_SPINLOCK().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dexuan Cui <decui@microsoft.com>
Fixes: bd1060a1 ("sock, cgroup: add sock->sk_cgroup")
Signed-off-by: David S. Miller <davem@davemloft.net>