s390: simplify memory notifier for protecting kdump crash kernel area

Assume we have a crashkernel area of 256MB reserved:

root@vm0:~# cat /proc/iomem
00000000-6fffffff : System RAM
  0f258000-0fcfffff : Kernel code
  0fd00000-101d10e3 : Kernel data
  105b3000-1068dfff : Kernel bss
70000000-7fffffff : Crash kernel

This exactly corresponds to memory block 7 (memory block size is 256MB).
Trying to offline that memory block results in:

root@vm0:~# echo "offline" > /sys/devices/system/memory/memory7/state
-bash: echo: write error: Device or resource busy

[  128.458762] page:000003d081c00000 refcount:1 mapcount:0 mapping:00000000d01cecd4 index:0x0
[  128.458773] flags: 0x1ffff00000001000(reserved)
[  128.458781] raw: 1ffff00000001000 000003d081c00008 000003d081c00008 0000000000000000
[  128.458781] raw: 0000000000000000 0000000000000000 ffffffff00000001 0000000000000000
[  128.458783] page dumped because: unmovable page

The craskernel area is marked reserved in the bootmem allocator. This
results in the memmap getting initialized (refcount=1, PG_reserved), but
the pages are never freed to the page allocator.

So these pages look like allocated pages that are unmovable (esp.
PG_reserved), and therefore, memory offlining fails early, when trying to
isolate the page range.

We only have to care about the exchange area, make that clear.

Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Philipp Rudo <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Michal Hocko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Borntraeger <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>

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