kdumpctl: fix target identification for systems without initrd
    
    We get following error on the systems that have everything built-in and no
    initrd is used.
    
    	Kernel dev name of /dev/root is not found.
    	Dump target /dev/root is probably not mounted.
    
    It happens because `df $path` gets /dev/root from /proc/self/mountinfo.
    
    Fix this by identifying real target device when `df $path` returns
    Filesystem as /dev/root.
    
    Reported-and-tested-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
    Signed-off-by: Pratyush Anand <panand@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>