x86, 64-bit: update address space documentation

Impact: documentation update

Commit a6523748
(paravirt/x86, 64-bit: move __PAGE_OFFSET to leave a space for hypervisor)
changed address space without changing the documentation.

Change it according to the code change -- direct mapping start:
ffff810000000000 => ffff880000000000 which gives 57 TiB, something
between 45 and 46 bits.
Signed-off-by: NJiri Slaby <jirislaby@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation/x86/x86_64/mm.txt

@@ -6,7 +6,7 @@ Virtual memory map with 4 level page tables:
 0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm
 hole caused by [48:63] sign extension
 ffff800000000000 - ffff80ffffffffff (=40 bits) guard hole
-ffff810000000000 - ffffc0ffffffffff (=46 bits) direct mapping of all phys. memory
+ffff880000000000 - ffffc0ffffffffff (=57 TB) direct mapping of all phys. memory
 ffffc10000000000 - ffffc1ffffffffff (=40 bits) hole
 ffffc20000000000 - ffffe1ffffffffff (=45 bits) vmalloc/ioremap space
 ffffe20000000000 - ffffe2ffffffffff (=40 bits) virtual memory map (1TB)