Add validator to update_floatingip_port_forwarding so codepath does not
attempt performing invalid database operation. With that, operation fails
right away, with a hint on the offending argument(s).
This is a backport that combines changes from 2 changes that go together:
https://review.opendev.org/#/c/738145/https://review.opendev.org/#/c/744993/
Change-Id: I8284b22c5d691bfd9eadeb8590c3d4b27d261b04
Closes-Bug: #1878299
(cherry picked from commit f379740348)
(cherry picked from commit 838399f0a4)