s390/mm: Fix DirectMap accounting

With uncoupling of physical and virtual address spaces population of
the identity mapping was changed to use the type POPULATE_IDENTITY
instead of POPULATE_DIRECT. This breaks DirectMap accounting:

> cat /proc/meminfo
DirectMap4k:       55296 kB
DirectMap1M:    18446744073709496320 kB

Adjust all locations of update_page_count() in vmem.c to use
POPULATE_IDENTITY instead of POPULATE_DIRECT as well. With this
accounting is correct again:

> cat /proc/meminfo
DirectMap4k:       54264 kB
DirectMap1M:     8334336 kB

Fixes: c98d2eca ("s390/mm: Uncouple physical vs virtual address spaces")
Cc: stable@vger.kernel.org
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>