net/mlx5e: Refactor neigh update infrastructure

Following patches in series implements route update which can cause encap
entries to migrate between routing devices. Consecutively, their parent
nhe's need to be also transferable between devices instead of having neigh
device as a part of their immutable key. Move neigh device from struct
mlx5_neigh to struct mlx5e_neigh_hash_entry and check that nhe and neigh
devices are the same in workqueue neigh update handler.

Save neigh net_device that can change dynamically in dedicated nhe->dev
field. With FIB event handler that is implemented in following patches
changing nhe->dev, NETEVENT_DELAY_PROBE_TIME_UPDATE handler can
concurrently access the nhe entry when traversing neigh list under rcu read
lock. Processing stale values in that handler doesn't change the handler
logic, so just wrap all accesses to the dev pointer in {WRITE|READ}_ONCE()
helpers.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

net/mlx5e: Add trace point for neigh update

Allow tracing neigh state during neigh update task that is executed on
workqueue and is scheduled by neigh state change event.

Usage example:
># cd /sys/kernel/debug/tracing
># echo mlx5:mlx5e_rep_neigh_update >> set_event
># cat trace
...
kworker/u48:7-2221 [009] ...1 1475.387435: mlx5e_rep_neigh_update:
netdev: ens1f0 MAC: 24:8a:07:9a:17:9a IPv4: 1.1.1.10 IPv6: ::ffff:1.1.1.10 neigh_connected=1

Added corresponding documentation in
Documentation/networking/device-driver/mellanox/mlx5.rst

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Dmytro Linkin <dmitrolin@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>