In Rev 1.511 I moved peer related checks from inside up_generate_updates()

In Rev 1.511 I moved peer related checks from inside up_generate_updates()
to rde_generate_updates() but forgot to add the same checks to
rde_softreconfig_out(). As a result a config reload could result in
prefixes for a not-negotiated address family to leak into the Adj-RIB-Out.
Adjust both rde_generate_updates() and rde_softreconfig_out() to use the
same filter logic.
Also adjust rde_up_flush_upcall() to pass the peer as argument instead of
using prefix_peer(). The up_generate_updates() call there does not need
any filtering since the walker runs on the Adj-RIB-Out and just withdraws
everything.
OK job@ on a previous diff