dracut-module-setup.sh: rework kdump_get_ip_route_field
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit 46542ccda5df42a8b07e1d7fee629374e51e732a
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Aug 4 02:59:43 2021 +0800
    
        dracut-module-setup.sh: rework kdump_get_ip_route_field
    
        Avoid duplicated echo / cut / grep call, just use sed.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>