kdump-lib.sh: fix arithmetic operation syntax
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit c0edb80b8f2d6b859d7f7118942e825d35be5c51
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Sep 8 13:31:31 2021 +0800
    
        kdump-lib.sh: fix arithmetic operation syntax
    
        Get rid of let, and remove useless '$' on arithmetic variables.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>