s390/mm: properly clear _PAGE_NOEXEC bit when it is not supported

On older HW or under a hypervisor, w/o the instruction-execution-
protection (IEP) facility, and also w/o EDAT-1, a translation-specification
exception may be recognized when bit 55 of a pte is one (_PAGE_NOEXEC).

The current code tries to prevent setting _PAGE_NOEXEC in such cases,
by removing it within set_pte_at(). However, ptep_set_access_flags()
will modify a pte directly, w/o using set_pte_at(). There is at least
one scenario where this can result in an active pte with _PAGE_NOEXEC
set, which would then lead to a panic due to a translation-specification
exception (write to swapped out page):

do_swap_page
  pte = mk_pte (with _PAGE_NOEXEC bit)
  set_pte_at   (will remove _PAGE_NOEXEC bit in page table, but keep it
                in local variable pte)
  vmf->orig_pte = pte (pte still contains _PAGE_NOEXEC bit)
  do_wp_page
    wp_page_reuse
      entry = vmf->orig_pte (still with _PAGE_NOEXEC bit)
      ptep_set_access_flags (writes entry with _PAGE_NOEXEC bit)

Fix this by clearing _PAGE_NOEXEC already in mk_pte_phys(), where the
pgprot value is applied, so that no pte with _PAGE_NOEXEC will ever be
visible, if it is not supported. The check in set_pte_at() can then also
be removed.

Cc: <stable@vger.kernel.org> # 4.11+
Fixes: 57d7f939 ("s390: add no-execute support")
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>