Fix an nvpair leak in vdev_geom_read_config().

Also change the behaviour slightly: instead of freeing "config" if the
last nvlist doesn't pass the tests, return the last config that did pass
those tests. This matches the comment at the beginning of the function.

PR: 230704
Diagnosed by: avg
Reviewed by: asomers, avg
Tested by: Mark Martinec <Mark.Martinec@ijs.si>
Approved by: re (gjb)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D17202