x86, kaslr: export offset in VMCOREINFO ELF notes

Include kASLR offset in VMCOREINFO ELF notes to assist in debugging.

[ hpa: pushing this for v3.14 to avoid having a kernel version with
  kASLR where we can't debug output. ]

Signed-off-by: Eugene Surovegin <surovegin@google.com>
Link: http://lkml.kernel.org/r/20140123173120.GA25474@www.outflux.net
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/machine_kexec_64.c

@@ -279,5 +279,7 @@ void arch_crash_save_vmcoreinfo(void)
 	VMCOREINFO_SYMBOL(node_data);
 	VMCOREINFO_LENGTH(node_data, MAX_NUMNODES);
 #endif
+	vmcoreinfo_append_str("KERNELOFFSET=%lx\n",
+			      (unsigned long)&_text - __START_KERNEL);
 }