net/mlx5: fs, an FTE should have no dests when deleted

When deleting an FTE it should have no dests, which means
fte->dests_size should be 0. Add a WARN_ON() to catch bugs
where the proper tracking wasn't done.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>