fix: proper return code for inst_multiple in dracut-init.sh

fix: proper return code for inst_multiple in dracut-init.sh

A test of the form

    if ! command; then
        _ret=$?
        ...
        return _ret
    fi

does not capture the return code of `command`, but the negation of the
return code, leaving _ret == 0. The test of this form in inst_multiple
has been refactored to capture and return the right value.