Unlike pxd_free_tlb(), the pxd_free() functions do not check for folded
page tables. This is not an issue so far, as those functions will actually
never be called, since no code will reach them when page tables are folded.

In order to avoid future issues, and to make the s390 code more similar to
other architectures, add mm_pxd_folded() checks, similar to how it is done
in pxd_free_tlb().

This was found by testing a patch from from Anshuman Khandual, which is
currently discussed on LKML ("mm/debug: Add tests validating architecture
page table helpers").
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>