bgpd: check rtt later after the real peer is known

bgpd: check rtt later after the real peer is known

OPEN message handler moves the connection from the temporary
"struct peer" (used to accept it) to the real "struct peer" based
on the configuration. RTT needs to be updated only to the real
struct peer, and this patch moves the RTT query to point where
realpeer is known.

Fixes: ef757700d0 "bgpd: allow using rtt in route-map's set metric"
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
(cherry picked from commit 0edba8b6ad9c83fa0a3cc58765fe9f123f4109ac)

Conflicts:
	bgpd/bgp_packet.c