s390/vmem: Fix null-pointer-arithmetic warning in vmem_map_init()

s390/vmem: Fix null-pointer-arithmetic warning in vmem_map_init()

Fixes the following clang warning introduced by commit d7bebcb4a898
("s390: Optimize __pa/__va when RANDOMIZE_IDENTITY_BASE is off")

arch/s390/mm/vmem.c:665:36: warning: performing pointer arithmetic on
a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
  665 |                 __set_memory_4k(__va(0), __va(0) + ident_map_size);
      |                                          ~~~~~~~ ^

Fixes: d7bebcb4a898 ("s390: Optimize __pa/__va when RANDOMIZE_IDENTITY_BASE is off")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501270309.HzsVNo3o-lkp@intel.com/
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>