package/zfs: replace PYTHON_VERSION_MAJOR

Replace PYTHON_VERSION_MAJOR by PYTHON3_VERSION_MAJOR now that python2
has been dropped. It should be noted that PYTHON_SITE_PKG was wrongly
set since the addition of the package in commit
4470bc99

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>