s390/mm: improve debugfs ptdump markers walking

This allows to print multiple markers when they happened to have the
same value.

...
0x001bfffff0100000-0x001c000000000000       255M PMD I
---[ Kasan Shadow End ]---
---[ vmemmap Area ]---
0x001c000000000000-0x001c000002000000        32M PMD RW X
...

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>