[POWERPC] DEBUG_PAGEALLOC for 64-bit

Here's an implementation of DEBUG_PAGEALLOC for 64 bits powerpc.
It applies on top of the 32 bits patch.

Unlike Anton's previous attempt, I'm not using updatepp. I'm removing
the hash entries from the bolted mapping (using a map in RAM of all the
slots). Expensive but it doesn't really matter, does it ? :-)

Memory hot-added doesn't benefit from this unless it's added at an
address that is below end_of_DRAM() as calculated at boot time.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

 arch/powerpc/Kconfig.debug      |    2
 arch/powerpc/mm/hash_utils_64.c |   84 ++++++++++++++++++++++++++++++++++++++--
 2 files changed, 82 insertions(+), 4 deletions(-)
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/Kconfig.debug

@@ -20,7 +20,7 @@ config DEBUG_STACK_USAGE
 
 config DEBUG_PAGEALLOC
         bool "Debug page memory allocations"
-        depends on DEBUG_KERNEL && !SOFTWARE_SUSPEND && PPC32
+        depends on DEBUG_KERNEL && !SOFTWARE_SUSPEND
         help
           Unmap pages from the kernel linear mapping after free_pages().
           This results in a large slowdown, but helps to find certain types