powerpc: Dynamically allocate slb_shadow from memblock

Currently, the slb_shadow buffer is our largest symbol:

  [jk@pablo linux]$ nm --size-sort -r -S obj/vmlinux | head -1
  c000000000da0000 0000000000040000 d slb_shadow

- we allocate 128 bytes per cpu; so 256k with NR_CPUS=2048. As we have
constant initialisers, it's allocated in .text, causing a larger vmlinux
image. We may also allocate unecessary slb_shadow buffers (> no. pacas),
since we use the build-time NR_CPUS rather than the run-time nr_cpu_ids.

We could move this to the bss, but then we still have the NR_CPUS vs
nr_cpu_ids potential for overallocation.

This change dynamically allocates the slb_shadow array, during
initialise_pacas(). At a cost of 104 bytes of text, we save 256k of
data:

  [jk@pablo linux]$ size obj/vmlinux{.orig,}
     text     data      bss       dec     hex	filename
  9202795  5244676  1169576  15617047  ee4c17	obj/vmlinux.orig
  9202899  4982532  1169576  15355007  ea4c7f	obj/vmlinux

Tested on pseries.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>