powerpc/mm: Lockless get_user_pages_fast() for 64-bit (v3)

Implement lockless get_user_pages_fast for 64-bit powerpc.

Page table existence is guaranteed with RCU, and speculative page references
are used to take a reference to the pages without having a prior existence
guarantee on them.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>