mkdumprd: remove an awk call in get_fs_size
    
    upstream: fedora
    resolves: bz2003832
    conflict: none
    
    commit c486b1fa3010c4c5f190ece9a2ed0a1430dfe7b5
    Author: Kairui Song <kasong@redhat.com>
    Date:   Tue Sep 7 14:42:30 2021 +0800
    
        mkdumprd: remove an awk call in get_fs_size
    
        By using `df --output=avail`, the awk call can be dropped.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>