filesystems/py-libzfs: fix the build on FreeBSD 15

There were two problems:

• Needed to backport a newer py-libzfs PR due to a libzfs API change
• Since 15.0 is now released, its OSVERSION reset to 1500000

PR: 291393
Sponsored by: ConnectWise
Approved by: Goran Mekić <meka@tilda.center> (maintainer)

(cherry picked from commit f3f63d8635c962ae38191a4774df55790e18946e)