bgpd: Convert the bgp_advertise_attr->adv to a fifo

bgpd: Convert the bgp_advertise_attr->adv to a fifo

BGP is storing outgoing updates in a couple of different
fifo's.  This is to ensure proper packet packing of
all bgp_dests that happen to use the same attribute.

How it's all put together currently:  On initial update
BGP walks through all the bgp_dest's in a table.  For each
path being sent a bgp_advertise is created.  This bgp_advertise
is placed in fifo order on the bgp_synchronize->update queue.
The bgp_advertise has a pointer to the bgp_advertise_attr which
is associated iwth the actual attribute that is being sent to
it's peer.  In turn this bgp_advertise is placed in a fifo off
of the bgp_advertise_attr structure.  As such as we have paths
that share an attribute, the path/dest is placed on the
bgp_syncrhonize->update fifo as well as being placed on the fifo
associated with the advertised attribute.

On actual creation of a packet.  The first item in the
bgp_synchronize->update fifo is popped.  The bgp_advertise_attr
pointer is grabbed, we fill out the nlri part of the bgp packet
and then walk the bgp_advertise_attr fifo to place paths/dests in
the packet.  As each path/dest is placed in the packet it is removed
from both the bgp_synchronize->update fifo and the bgp_advertise_attr
fifo.

The whole point of this change is to switch the *next, *prev
pointers in the bgp_advertise structure with a typesafe data
structure.

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