s390/mm,ptdump: add proper ifdefs

Use ifdefs instead of IS_ENABLED() to avoid compile error
for !PTDUMP_DEBUGFS:

arch/s390/mm/dump_pagetables.c: In function ‘pt_dump_init’:
arch/s390/mm/dump_pagetables.c:248:64: error: ‘ptdump_fops’ undeclared (first use in this function); did you mean ‘pidfd_fops’?
   debugfs_create_file("kernel_page_tables", 0400, NULL, NULL, &ptdump_fops);

Reported-by: Julian Wiedmann <jwi@linux.ibm.com>
Fixes: 08c8e685 ("s390: add ARCH_HAS_DEBUG_WX support")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>