net/mlx5: E-Switch, Set correctly vport destination

The cited commit moved from using reformat_id integer to packet_reformat
pointer which introduced the possibility to null pointer dereference.
When setting packet reformat flag and pkt_reformat pointer must
exists so checking MLX5_ESW_DEST_ENCAP is not enough, we need
to make sure the pkt_reformat is valid and check for MLX5_ESW_DEST_ENCAP_VALID.
If the dest encap valid flag does not exists then pkt_reformat can be
either invalid address or null.
Also, to make sure we don't try to access invalid pkt_reformat set it to
null when invalidated and invalidate it before calling add flow code as
its logically more correct and to be safe.

Fixes: 2b688ea5 ("net/mlx5: Add flow steering actions to fs_cmd shim layer")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Chris Mi <cmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>