net/mlx5: Allow blocking encap changes in eswitch

Existing eswitch encap option enables header encapsulation. Unfortunately
currently available hardware isn't able to perform double encapsulation,
which can happen once IPsec packet offload tunnel mode is used together
with encap mode set to BASIC.

So as a solution for misconfiguration, provide an option to block encap
changes, which will be used for IPsec packet offload.

Reviewed-by: Emeel Hakim <ehakim@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>