s390/mm: Get rid of RELOC_HIDE()

Since __va(0) does not translate to NULL anymore remove RELOC_HIDE()
which was only added to get rid of a compile warning with clang W=1:

    arch/s390/mm/vmem.c:666:36: warning: performing pointer arithmetic on
     a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
      666 |                 __set_memory_4k(__va(0), __va(0) + ident_map_size);
          |                                          ~~~~~~~ ^

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>