x86/kdump/64: Restrict kdump kernel reservation to <64TB

Restrict kdump to only reserve crashkernel below 64TB.

The reaons is that the kdump may jump from a 5-level paging mode to a
4-level paging mode kernel. If a 4-level paging mode kdump kernel is put
above 64TB, then the kdump kernel cannot start.

The 1st kernel reserves the kdump kernel region during bootup. At that
point it is not known whether the kdump kernel has 5-level or 4-level
paging support.

To support both restrict the kdump kernel reservation to the lower 64TB
address space to ensure that a 4-level paging mode kdump kernel can be
loaded and successfully started.

[ tglx: Massaged changelog ]

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Dave Young <dyoung@redhat.com>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20190524073810.24298-4-bhe@redhat.com