bgpd: use DLIST for adv_fifo

bgpd: use DLIST for adv_fifo

I mistakenly assumed that something called "FIFO" would primarily be
used by removing items from the front.  This isn't the case for the
adv_fifo... so use a DLIST there to make sure deletions from the middle
of the queue are fast.

Signed-off-by: David Lamparter <equinox@diac24.net>