bgpd: Add peers back to peer hash when peer_xfer_conn fails

bgpd: Add peers back to peer hash when peer_xfer_conn fails

It was noticed that occassionally peering failed in a testbed
upon investigation it was found that the peer was not in the
peer hash and we saw these failure messages:

Aug 25 21:31:15 doca-hbn-service-bf3-s06-1-ipmi bgpd[3048]: %NOTIFICATION: sent to neighbor 2001:cafe:1ead:4::4 4/0 (Hold Timer Expired) 0 bytes
Aug 25 21:31:22 doca-hbn-service-bf3-s06-1-ipmi bgpd[3048]: [EC 100663299] Can't get remote address and port: Transport endpoint is not connected
Aug 25 21:31:22 doca-hbn-service-bf3-s06-1-ipmi bgpd[3048]: [EC 100663299] %bgp_getsockname() failed for  peer 2001:cafe:1ead:4::4 fd 27 (from_peer fd -1)
Aug 25 21:31:22 doca-hbn-service-bf3-s06-1-ipmi bgpd[3048]: [EC 33554464] %Neighbor failed in xfer_conn

root@doca-hbn-service-bf3-s06-1-ipmi:/var/log/hbn/frr# vtysh -c 'show bgp peerhash' | grep 2001:cafe:1ead:4::4
root@doca-hbn-service-bf3-s06-1-ipmi:/var/log/hbn/frr#

Upon looking at the code the peer_xfer_conn function can fail
and the bgp_establish code will then return before adding the
peer back to the peerhash.

This is only part of the failure.  The peer also appears to
be in a state where it is no longer initiating connection attempts
but that will be another commited fix when we figure that one out.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>