s390/mm: fix dump_pagetables top level page table walking

Since commit d1874a0c ("s390/mm: make the pxd_offset functions more
robust") behaviour of p4d_offset, pud_offset and pmd_offset has been
changed so that they cannot be used to iterate through top level page
table, because the index for the top level page table is now calculated
in pgd_offset. To avoid dumping the very first region/segment top level
table entry 2048 times simply iterate entry pointer like it is already
done in other page walking cases.

Fixes: d1874a0c ("s390/mm: make the pxd_offset functions more robust")
Reported-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>