zfs-kmods: install to /lib/modules instead of /usr/lib/modules

zfs-kmods: install to /lib/modules instead of /usr/lib/modules

Before this patch, dracut wouldn't find zfs.ko for inclusion in
initramfs. This was caused by the packages installing in to
/lib/modules instead of /usr/lib/modules.  Correcting this allows
dracut to do the right thing, even without

    # /etc/dracut.conf
    add_drivers+=" zfs "

Notably, rpm/redhat/zfs-kmod.spec.in does not contain the definition of
the `prefix` macro that this commit removes in the generic kmod spec.
And https://rpmfusion.org/Packaging/KernelModules/Kmods2 does not
mention `prefix` at all.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11381