bash scripts: use $(...) notation instead of legacy `...`
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit 54cc5c44befa308e122d93221c65486164ffb3e5
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Sep 8 01:48:52 2021 +0800
    
        bash scripts: use $(...) notation instead of legacy `...`
    
        This is a batch update done with following command:
    
        `sed -i -e 's/`\([^`]*\)`/\$(\1)/g' mkfadumprd mkdumprd \
         kdumpctl dracut-module-setup.sh dracut-fadump-module-setup.sh \
         dracut-early-kdump-module-setup.sh`
    
        And manually converted some corner cases. This fixes
        all related issues detected by shellcheck.
        Make it easier to do clean up in later commits.
    
        Check following link for reasons to switch to the new syntax:
        https://github.com/koalaman/shellcheck/wiki/SC2006
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>