kdumpctl: refine grep usage
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit 80525afaceac3fa6bdf6e57686f66ab497d2f153
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Aug 4 15:44:02 2021 +0800
    
        kdumpctl: refine grep usage
    
        Use `grep -q` instead of redirect to /dev/null.
    
        Use `grep -c` instead, as suggested in:
        https://github.com/koalaman/shellcheck/wiki/SC2126
    
        Use `grep -E` instead of `egrep`.
        https://github.com/koalaman/shellcheck/wiki/SC2196
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>