s390/vmem: fix vmem_add_range for 4-level paging

The kernel currently crashes if 4-level paging is used. Add missing
p4d_populate for just allocated pud entry.

Fixes: 3e0d3e40 ("s390/vmem: consolidate vmem_add_range() and vmem_remove_range()")
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>