x86/mm: Fix documentation of module mapping range with 4-level paging

Commit:

f5a40711fa58 ("x86/mm: Set MODULES_END to 0xffffffffff000000")

changed MODULES_END back to a fixed value, but didn't update the documentation
of memory layout for 4-level paging.

Signed-off-by: Kirill A. Shutemov <[e-mail]>
Acked-by: Andrey Ryabinin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Fixes: f5a40711fa58 ("x86/mm: Set MODULES_END to 0xffffffffff000000")
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

Documentation/x86/x86_64/mm.txt

···
 ffffffef00000000 - fffffffeffffffff (=64 GB) EFI region mapping space
 ... unused hole ...
 ffffffff80000000 - ffffffff9fffffff (=512 MB)  kernel text mapping, from phys 0
-ffffffffa0000000 - [fixmap start]   (~1526 MB) module mapping space (variable)
+ffffffffa0000000 - fffffffffeffffff (1520 MB) module mapping space
 [fixmap start]   - ffffffffff5fffff kernel-internal fixmap range
 ffffffffff600000 - ffffffffff600fff (=4 kB) legacy vsyscall ABI
 ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole