lib: fix prefix-list duplication check

lib: fix prefix-list duplication check

Currently, when we check the new prefix-list entry for duplication, we
only take filled in fields into account and ignore optional fields.
For example, if we already have `ip prefix-list A 0.0.0.0/0 le 32` and
we try to add `ip prefix-list A 0.0.0.0/0`, it is treated as duplicate.
We should always compare all prefix-list fields when doing the check.

Fixes #9355.

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