kdump-lib.sh: fix a few ambiguous or redundant code
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit 319219d23b003310f457e3ba549bf0fa0254d7da
    Author: Kairui Song <kasong@redhat.com>
    Date:   Mon Sep 13 01:17:23 2021 +0800
    
        kdump-lib.sh: fix a few ambiguous or redundant code
    
        Fix a few ambiguous syntax issues and remove some unused variables.
        Also refactor some code to make it more robust.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>