net/mlx5e: Read neigh parameters with proper locking

The nud_state and hardware address fields are protected by the neighbour
lock, we should acquire it before accessing those parameters.

Use this lock to avoid inconsistency between the neighbour validity state
and it's hardware address.

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