net/mlx5: E-Switch, Fix uninitialized variable modact

commit d8e5883d upstream.

The variable modact is not initialized before used in command
modify header allocation which can cause command to fail.

Fix by initializing modact with zeros.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 8f1e0b97

 ("net/mlx5: E-Switch, Mark miss packets with new chain id mapping")
Signed-off-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>