kdumpctl: Add kdumpctl reset-crashkernel
    
    Resolves: bz1974638
    Upstream: Fedora
    Conflict: None
    
    commit 86130ec10fda37cc283f717eaacb56a4cbf76418
    Author: Kairui Song <kasong@redhat.com>
    Date:   Thu Jun 10 13:06:22 2021 +0800
    
        kdumpctl: Add kdumpctl reset-crashkernel
    
        In newer kernel, crashkernel.default will contain the default
        crashkernel value of a kernel build. So introduce a new sub command
        to help user reset kernel crashkernel size to the default value.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Baoquan He <bhe@redhat.com>
    
    Signed-off-by: Kairui Song <kasong@redhat.com>