net/mlx5: fix tunnel offload hub multi-thread protection

net/mlx5: fix tunnel offload hub multi-thread protection

The original patch was removing active tunnel offload objects from a
tunnels db list without checking its reference counter value.
That action was leading to a PMD crash.

Current patch isolates tunnels db list into a separate API. That API
manages MT protection of the tunnel offload db.

Fixes: 5b38d8cd4663 ("net/mlx5: make tunnel hub list thread safe")

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>