RDMA/mlx5: Fix vport loopback forcing for MPV device

Previously loopback for MPV was supposed to be permanently enabled,
however other driver flows were able to over-ride that configuration and
disable it.

Add force_lb parameter that indicates that loopback should always be
enabled which prevents all other driver flows from disabling it.

Fixes: a9a9e689 ("RDMA/mlx5: Fix vport loopback for MPV device")
Link: https://patch.msgid.link/r/cfc6b1f0f99f8100b087483cc14da6025317f901.1755088808.git.leon@kernel.org


Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>