powerpc/mm: Fix no execute fault handling on pre-POWER5

Aneesh/Ben reported that the change to do_page_fault() we made in commit
1d18ad02 ("powerpc/mm: Detect instruction fetch denied and report")
needs to handle the case where CPU_FTR_COHERENT_ICACHE is missing but we
have CPU_FTR_NOEXECUTE. In those cases the check added for
SRR1_ISI_N_OR_G might trigger a false positive.

This patch adds a check for CPU_FTR_COHERENT_ICACHE in addition to the
MSR value.

Fixes: 1d18ad02 ("powerpc/mm: Detect instruction fetch denied and report")
Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>