bash scripts: get rid of expr and let
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit c4d85142be300814b8818fe8aad65eaa042e01c8
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Aug 4 15:18:59 2021 +0800
    
        bash scripts: get rid of expr and let
    
        As suggested by:
        https://github.com/koalaman/shellcheck/wiki/SC2219
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>