use pointer arithmetics for dump_dmesg
    
    Resolves: bz2069200
    Upstream: github.com/makedumpfile/makedumpfile.git
    Conflicts: None
    
    commit e1d2e5302b016c6f7942f46ffa27aa31326686c5
    Author: Philipp Rudo <prudo@redhat.com>
    Date:   Mon Mar 14 17:04:30 2022 +0100
    
        [PATCH v2 2/3] use pointer arithmetics for dump_dmesg
    
        When parsing the printk buffer for the old printk mechanism (> v3.5.0+ and
        < 5.10.0) a log entry is currently specified by the offset into the
        buffer where the entry starts. Change this to use a pointers instead.
        This is done in preparation for using the new cycle detection mechanism.
    
        Signed-off-by: Philipp Rudo <prudo@redhat.com>
        Reviewed-and-tested-by: Dave Wysochanski <dwysocha@redhat.com>
        Signed-off-by: Kazuhito Hagio <k-hagio-ab@nec.com>
    
    Signed-off-by: Philipp Rudo <prudo@redhat.com>