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 <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Reviewed-by: Heiko Carstens <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>
Signed-off-by: Alexander Gordeev <[e-mail]>

arch/s390/mm/vmem.c

···
 	if (!static_key_enabled(&cpu_has_bear))
 		set_memory_x(0, 1);
 	if (debug_pagealloc_enabled())
-		__set_memory_4k(__va(0), __va(0) + ident_map_size);
+		__set_memory_4k(__va(0), absolute_pointer(__va(0)) + ident_map_size);
 	pr_info("Write protected kernel read-only data: %luk\n",
 		(unsigned long)(__end_rodata - _stext) >> 10);
 }