net/mlx5e: Add refcount to VXLAN structure

[ Upstream commit 23f4cc2c ]

A refcount mechanism must be implemented in order to prevent unwanted
scenarios such as:
- Open an IPv4 VXLAN interface
- Open an IPv6 VXLAN interface (different socket)
- Remove one of the interfaces

With current implementation, the UDP port will be removed from our VXLAN
database and turn off the offloads for the other interface, which is
still active.
The reference count mechanism will only allow UDP port removals once all
consumers are gone.

Fixes: b3f63c3d ("net/mlx5e: Add netdev support for VXLAN tunneling")
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>