powerpc/mm: Fix lazy icache flush on pre-POWER5

On 64-bit CPUs with no-execute support and non-snooping icache, such as
970 or POWER4, we have a software mechanism to ensure coherency of the
cache (using exec faults when needed).

This was broken due to a logic error when the code was rewritten
from assembly to C, previously the assembly code did:

  BEGIN_FTR_SECTION
         mr      r4,r30
         mr      r5,r7
         bl      hash_page_do_lazy_icache
  END_FTR_SECTION(CPU_FTR_NOEXECUTE|CPU_FTR_COHERENT_ICACHE, CPU_FTR_NOEXECUTE)

Which tests that:
   (cpu_features & (NOEXECUTE | COHERENT_ICACHE)) == NOEXECUTE

Which says that the current cpu does have NOEXECUTE, but does not have
COHERENT_ICACHE.

Fixes: 91f1da99 ("powerpc/mm: Convert 4k hash insert to C")
Fixes: 89ff7250 ("powerpc/mm: Convert __hash_page_64K to C")
Fixes: a43c0eb8 ("powerpc/mm: Convert 4k insert from asm to C")
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[mpe: Change log verbosification]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>