get_persistent_dev(): fix name contention with dracut's similar function
    
    Resolves: BZ1348898
    
    dracut-functions.sh defines a get_persistent_dev(). Earlier, we had another
    local get_persistent_dev() in mkdumprd, however that was moved to
    kdump-lib.sh, so that it can be reused in kdumpctl.
    
    Since, dracut-module-setup.sh (which is dracut's
    99kdumpbase/module-setup.sh) sources kdump-lib.sh. Therefore, once dracut
    will execute 99kdumpbase module, it's own get_persistent_dev() function is
    overwritten by kdump's version. If any other dracut module calls
    get_persistent_dev() thereafter then, kdump's version is executed, which was
    not expected.
    
    Therefore rename kdump's get_persistent_dev() as kdump_get_persistent_dev()
    to avoid any name contention.
    
    Signed-off-by: Pratyush Anand <panand@redhat.com>
    Acked-by: Xunlei Pang <xlpang@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>