Cause zfs.spec to place dracut files properly

This is an extension of @behlendorf's commit https://github.com/zfsonlinux/zfs/commit/ffb21118add1a2e6cec2ce401c333b4a4e76d9a3

As the fedora conditional has been added, this allows centos/rhel-6 to fall back to the proper directory (/usr/share/dracut)