net/mlx5e: Pass attr arg for attaching/detaching encaps

In later commit that we will have multiple attr instances per flow
we would like to pass a specific attr instance to set encaps.

Currently the mlx5_flow object contains a single mlx5_attr instance.
However, multi table actions (e.g. CT) instantiate multiple attr instances.

Currently mlx5e_attach/detach_encap() reads the first attr instance
from the flow instance. Modify the functions to receive the attr
instance as a parameter which is set by the calling function.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>