kdump-lib.sh: introduce functions to return recommened mem size
    
    Resolves: bz1892558
    Upstream: Fedora
    Conflict: None
    
    commit 18131894b609eb233496af81d034d65c96d4d0e6
    Author: Pingfan Liu <piliu@redhat.com>
    Date:   Thu Feb 4 09:45:36 2021 +0800
    
        kdump-lib.sh: introduce functions to return recommened mem size
    
        There is requirement to decide the recommended memory size for the current
        system. And the algorithm is based on /proc/iomem, so it can align with the
        algorithm used by reserve_crashkernel() in kernel.
    
        Signed-off-by: Pingfan Liu <piliu@redhat.com>
        Acked-by: Kairui Song <kasong@redhat.com>
    
    Signed-off-by: Pingfan Liu <piliu@redhat.com>