print kdump kernel loaded status in stack dump

It is useful to print kdump kernel loaded status in dump_stack()
especially when panic happens so that we can differenciate
kdump kernel early hang and a normal panic in a bug report.

Link: http://lkml.kernel.org/r/20180127041129.GA29016@dhcp-128-65.nay.redhat.com
To: Steven Rostedt <rostedt@goodmis.org>
To: Andi Kleen <ak@linux.intel.com>
To: kexec@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: akpm@linux-foundation.org
Cc: kexec@lists.infradead.org
Signed-off-by: Dave Young <dyoung@redhat.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Petr Mladek <pmladek@suse.com>