powerpc/mm/hash64: Fix subpage protection with 4K HPTE config

commit aac55d7573c5d46ed9a62818d5d3e69dd2060105 upstream.

With Linux page size of 64K and hardware only supporting 4K HPTE, if we
use subpage protection, we always fail for the subpage 0 as shown
below (using the selftest subpage_prot test):

  520175565:  (4520111850): Failed at 0x3fffad4b0000 (p=13,sp=0,w=0), want=fault, got=pass !
  4520890210: (4520826495): Failed at 0x3fffad5b0000 (p=29,sp=0,w=0), want=fault, got=pass !
  4521574251: (4521510536): Failed at 0x3fffad6b0000 (p=45,sp=0,w=0), want=fault, got=pass !
  4522258324: (4522194609): Failed at 0x3fffad7b0000 (p=61,sp=0,w=0), want=fault, got=pass !

This is because hash preload wrongly inserts the HPTE entry for subpage
0 without looking at the subpage protection information.

Fix it by teaching should_hash_preload() not to preload if we have
subpage protection configured for that range.

It appears this has been broken since it was introduced in 2008.

Fixes: fa28237cfcc5 ("[POWERPC] Provide a way to protect 4k subpages when using 64k pages")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[mpe: Rework into should_hash_preload() to avoid build fails w/SLICES=n]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>