x86, kdump: Remove crashkernel range find limit for 64bit

Now kexeced kernel/ramdisk could be above 4g, so remove 896 limit for
64bit.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-30-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>