net/mlx5e: Extend hairpin entry with reference counter

List of flows attached to hairpin entry is used as implicit reference
counter (hairpin entry is deallocated when list becomes free) and as a
mechanism to obtain hairpin entry that flow is attached to (through list
head). This is not safe when concurrent modification of list of flows
attached to hairpin entry is possible. Proper atomic reference counter is
required to support concurrent access.

As a preparation for extending hairpin with reference counting, extract
code that deletes hairpin entry into standalone function. In order to
remove this dependency on external locking, extend hairpin entry with
reference counter to manage its lifetime and extend flow structure with
direct pointer to hairpin entry that flow is attached to.

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>