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 <[email protected]>
Signed-off-by: Christian Schwarz <[email protected]>
Closes #11381

rpm/generic/zfs-kmod.spec.in

@@ -88,10 +88,6 @@ BuildRequires:  %{_bindir}/kmodtool
 %global __global_ldflags %{nil}
 %endif
 
-%if 0%{?fedora} >= 17
-%define prefix  /usr
-%endif
-
 # Kmodtool does its magic here.  A patched version of kmodtool is shipped
 # with the source rpm until kmod development packages are supported upstream.
 # https://bugzilla.rpmfusion.org/show_bug.cgi?id=2714