net/mlx5e: Add support to neighbour update flow

In order to offload TC encap rules, the driver does a lookup for the IP
tunnel neighbour according to the output device and the destination IP
given by the user.

To keep tracking after the validity state of such neighbours, we keep
the neighbours information (pair of device pointer and destination IP)
in a hash table maintained at the relevant egress representor and
register to get NETEVENT_NEIGH_UPDATE events. When getting neighbour update
netevent, we search for a match among the cached neighbours entries used for
encapsulation.

In case the neighbour isn't valid, we can't offload the flow into the
HW. We cache the flow (requested matching and actions) in the driver and
offload the rule later, when the neighbour is resolved and becomes
valid.

When a flow is only cached in the driver and not offloaded into HW
yet, we use EAGAIN return value to mark it internally, the TC ndo still
returns success.

Listen to kernel neighbour update netevents to trace relevant neighbours
validity state:

1. If a neighbour becomes valid, offload the related rules to HW.

2. If the neighbour becomes invalid, remove the related rules from HW.

3. If the neighbour mac address was changed, update the encap header.
   Remove all the offloaded rules using the old encap header from the HW
   and insert new rules to HW with updated encap header.

Access to the neighbors hash table is protected by RTNL lock of its
caller or by the table's spinlock.

Details of the locking/synchronization among the different actions
applied on the neighbour table:

Add/remove operations - protected by RTNL lock of its caller (all TC
commands are protected by RTNL lock). Add and remove operations are
initiated only when the user inserts/removes a TC rule into/from the driver.

Lookup/remove operations - since the lookup operation is done from
netevent notifier block, RTNL lock can't be used (atomic context).
Use the table's spin lock to protect lookups from TC user removal operation.
bh is used since netevent can be called from a softirq context.

Lookup/add operations - The hash table access functions are taking
care of the protection between lookup and add operations.

When adding/removing encap headers and rules to/from the HW, RTNL lock
is used. It can happen when:

1. The user inserts/removes a TC rule into/from the driver (TC commands
are protected by RTNL lock of it's caller).

2. The driver gets neighbour notification event, which reports about
neighbour validity status change. Before adding/removing encap headers
and rules to/from the HW, RTNL lock is taken.

A neighbour hash table entry should be freed when its encap list is empty.
Since The neighbour update netevent notification schedules a neighbour
update work that uses the neighbour hash entry, it can't be freed
unconditionally when the encap list becomes empty during TC delete rule flow.
Use reference count to protect from freeing neighbour hash table entry
while it's still in use.

When the user asks to unregister a netdvice used by one of the neigbours,
neighbour removal notification is received. Then we take a reference on the
neighbour and don't free it until the relevant encap entries (and flows) are
marked as invalid (not offloaded) and removed from HW.
As long as the encap entry is still valid (checked under RTNL lock) we
can safely access the neighbour device saved on mlx5e_neigh struct.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>