net/mlx5: E-Switch, Fix access to invalid memory when toggling esw modes

The esw fdb table has a union of legacy and offloads members.
So if we were in a certain esw mode we could set some memebers and not
set null which is fine as on destroy path and don't care.
But then moving from legacy to switchdev a second time, the cleanup flow
of legacy mode checks if a struct member was in use if it's not null so
we need to make sure to reset the code to null when we init legacy mode.

Fixes: 8da202b2 ("net/mlx5: E-Switch, Add support for VEPA in legacy mode.")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Huy Nguyen <huyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

drivers/net/ethernet/mellanox/mlx5/core/eswitch.c

@@ -431,6 +431,8 @@ static int esw_create_legacy_table(struct mlx5_eswitch *esw)
 {
 	int err;
 
+	memset(&esw->fdb_table.legacy, 0, sizeof(struct legacy_fdb));
+
 	err = esw_create_legacy_vepa_table(esw);
 	if (err)
 		return err;