bgpd: use double-linked list instead of single-linked list in dampening

bgpd: use double-linked list instead of single-linked list in dampening

This code always used double-linked list before this rework in 8.0 that
introduced all these crashes and memory leaks. Using single-linked list
is actually a performance regression, because there are frequent removes
here and single-linked list obviously handles removes much worse.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>