mptcp: fix local endpoint accounting

In mptcp_pm_nl_rm_addr_or_subflow() we always mark as available
the id corresponding to the just removed address.

The used bitmap actually tracks only the local IDs: we must
restrict the operation when a (local) subflow is removed.

Fixes: a88c9e4969 ("mptcp: do not block subflows creation on errors")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/pm_netlink.c

@@ -807,7 +807,8 @@ static void mptcp_pm_nl_rm_addr_or_subflow(struct mptcp_sock *msk,
 			removed = true;
 			__MPTCP_INC_STATS(sock_net(sk), rm_type);
 		}
-		__set_bit(rm_list->ids[i], msk->pm.id_avail_bitmap);
+		if (rm_type == MPTCP_MIB_RMSUBFLOW)
+			__set_bit(rm_list->ids[i], msk->pm.id_avail_bitmap);
 		if (!removed)
 			continue;