s390/mm/ptdump: Improve sorting of markers

Use the sort() from lib/sort.c to sort markers instead of the private
implementation. The current implementation does not sort markers
properly if they have to be moved downwards:

---[ Real Memory Copy Area Start ]---
0x0000035b903ff000-0x0000035b90400000         4K PTE I
---[ vmalloc Area Start ]---
---[ Real Memory Copy Area End ]---

Add a new member to each marker which indicates if a marker is start
of an area. If addresses of areas are equal consider an address which
defines the start of an area higher than the address which defines the
end of an area. In result the output is sorted as intended:

---[ Real Memory Copy Area Start ]---
0x0000019cedcff000-0x0000019cedd00000         4K PTE I
---[ Real Memory Copy Area End ]---
---[ vmalloc Area Start ]---

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>