net: core: protect rate estimator statistics pointer with lock

Extend gen_new_estimator() to also take stats_lock when re-assigning rate
estimator statistics pointer. (to be used by unlocked actions)

Rename 'stats_lock' to 'lock' and change argument description to explain
that it is now also used for control path.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>