mptcp: allow changing the "backup" bit by endpoint id

a non-zero 'id' is sufficient to identify MPTCP endpoints: allow changing
the value of 'backup' bit by simply specifying the endpoint id.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/158


Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>