mptcp: allow privileged operations from user namespaces

GENL_ADMIN_PERM checks that the user has CAP_NET_ADMIN in the initial
namespace by calling netlink_capable(). Instead, use GENL_UNS_ADMIN_PERM
which uses netlink_ns_capable(). This checks that the caller has
CAP_NET_ADMIN in the current user namespace.

See also

  commit 4a92602a ("openvswitch: allow management from inside user namespaces")

which introduced this mechanism. See also

  commit 5617c6cd ("nl80211: Allow privileged operations from user namespaces")

which introduced this for nl80211.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>