mptcp: add mptcp_for_each_subflow_safe helper

Similar to mptcp_for_each_subflow(): this is clearer now that the _safe
version is used in multiple places.

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>