powerpc/mm: Fix pmd/pte_devmap() on non-leaf entries

The Radix MMU translation tree as defined in ISA v3.0 contains two
different types of entry, directories and leaves. Leaves are
identified by _PAGE_PTE being set.

The formats of the two entries are different, with the directory
entries containing no spare bits for use by software. In particular
the bit we use for _PAGE_DEVMAP is not reserved for software, and is
part of the NLB (Next Level Base) field, essentially the address of
the next level in the tree.

Note that the Linux pte_t is not == _PAGE_PTE. A huge page pmd
entry (or devmap!) is also a leaf and so has _PAGE_PTE set, even
though we use a pmd_t for it in Linux.

The fix is to ensure that the pmd/pte_devmap() confirm they are
looking at a leaf entry (_PAGE_PTE) as well as checking _PAGE_DEVMAP.

Fixes: ebd31197

 ("powerpc/mm: Add devmap support for ppc64")
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Tested-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
Reviewed-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
[mpe: Add a comment in the code and flesh out change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>