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)
MFH: 2025Q4