add slurp_proc_file()
    
    Resolves: bz2060824
    Upstream: git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
    Conflicts: None
    
    commit 193e51deccc62544f6423eb5e5eefc8a23aad679
    Author: Sven Schnelle <svens@linux.ibm.com>
    Date:   Thu Dec 16 12:43:54 2021 +0100
    
        add slurp_proc_file()
    
        slurp_file() cannot be used to read proc files, as they are returning
        a size of zero in stat(). Add a function slurp_proc_file() which is
        similar to slurp_file(), but doesn't require the size of the file to
        be known.
    
        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>