powerpc/mm: move setting pte specific flags to pfn_pte

powerpc used to set the pte specific flags in set_pte_at().  This is
different from other architectures.  To be consistent with other
architecture update pfn_pte to set _PAGE_PTE on ppc64.  Also, drop now
unused pte_mkpte.

We add a VM_WARN_ON() to catch the usage of calling set_pte_at() without
setting _PAGE_PTE bit.  We will remove that after a few releases.

With respect to huge pmd entries, pmd_mkhuge() takes care of adding the
_PAGE_PTE bit.

[akpm@linux-foundation.org: whitespace fix, per Christophe]

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lkml.kernel.org/r/20200902114222.181353-3-aneesh.kumar@linux.ibm.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>