mm/debug_vm_pgtable/set_pte/pmd/pud: don't use set_*_at to update an existing pte entry

set_pte_at() should not be used to set a pte entry at locations that
already holds a valid pte entry.  Architectures like ppc64 don't do TLB
invalidate in set_pte_at() and hence expect it to be used to set locations
that are not a valid PTE.

Link: https://lkml.kernel.org/r/20200902114222.181353-8-aneesh.kumar@linux.ibm.com


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