RDMA/mlx5: Check CAP_NET_RAW in user namespace for anchor create

RDMA/mlx5: Check CAP_NET_RAW in user namespace for anchor create

Currently, the capability check is done in the default
init_user_ns user namespace. When a process runs in a
non default user namespace, such check fails. Due to this
when a process is running using Podman, it fails to create
the anchor.

Since the RDMA device is a resource within a network namespace,
use the network namespace associated with the RDMA device to
determine its owning user namespace.

Fixes: 0c6ab0ca9a66 ("RDMA/mlx5: Expose steering anchor to userspace")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Link: https://patch.msgid.link/c2376ca75e7658e2cbd1f619cf28fbe98c906419.1750963874.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>