s390/mm: fix set_huge_pte_at() for empty ptes

commit ac8372f3

 upstream.

On s390, the layout of normal and large ptes (i.e. pmds/puds) differs.
Therefore, set_huge_pte_at() does a conversion from a normal pte to
the corresponding large pmd/pud. So, when converting an empty pte, this
should result in an empty pmd/pud, which would return true for
pmd/pud_none().

However, after conversion we also mark the pmd/pud as large, and
therefore present. For empty ptes, this will result in an empty pmd/pud
that is also marked as large, and pmd/pud_none() would not return true.

There is currently no issue with this behaviour, as set_huge_pte_at()
does not seem to be called for empty ptes. It would be valid though, so
let's fix this by not marking empty ptes as large in set_huge_pte_at().

This was found by testing a patch from from Anshuman Khandual, which is
currently discussed on LKML ("mm/debug: Add more arch page table helper
tests").

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>