net/mlx5e: Protect neigh hash encap list with spinlock and rcu

Rcu-ify mlx5e_neigh_hash_entry->encap_list by changing operations on encap
list to their rcu counterparts and extending encap structure with rcu_head
to free the encap instances after rcu grace period. Use rcu read lock when
traversing encap list. Implement helper mlx5e_get_next_valid_encap()
function that is used by mlx5e_tc_update_neigh_used_value() to safely
iterate over valid entries of nhe->encap_list.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Jianbo Liu <jianbol@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>