net/mlx5e: Check the number of elements before walk TC rhashtable

net/mlx5e: Check the number of elements before walk TC rhashtable

After IPSec TX tables are destroyed, the flow rules in TC rhashtable,
which have the destination to IPSec, are restored to the original
one, the uplink.

However, when the device is in switchdev mode and unload driver with
IPSec rules configured, TC rhashtable cleanup is done before IPSec
cleanup, which means tc_ht->tbl is already freed when walking TC
rhashtable, in order to restore the destination. So add the checking
before walking to avoid unexpected behavior.

Fixes: d1569537a837 ("net/mlx5e: Modify and restore TC rules for IPSec TX rules")
Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>