use slurp_proc_file() in get_command_line()
    
    Resolves: bz2060824
    Upstream: git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
    Conflicts: None
    
    commit d6516ba4c88f217fe14455db92c60cd0e9af18f8
    Author: Sven Schnelle <svens@linux.ibm.com>
    Date:   Thu Dec 16 12:43:55 2021 +0100
    
        use slurp_proc_file() in get_command_line()
    
        This way the size of the command line that get_command_line() can handle
        is no longer fixed.
    
        Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
        Signed-off-by: Simon Horman <horms@verge.net.au>
    
    Signed-off-by: Philipp Rudo <prudo@redhat.com>