device-dax: "Hotremove" persistent memory that is used like normal RAM

It is now allowed to use persistent memory like a regular RAM, but
currently there is no way to remove this memory until machine is
rebooted.

This work expands the functionality to also allows hotremoving
previously hotplugged persistent memory, and recover the device for use
for other purposes.

To hotremove persistent memory, the management software must first
offline all memory blocks of dax region, and than unbind it from
device-dax/kmem driver.  So, operations should look like this:

  echo offline > /sys/devices/system/memory/memoryN/state
  ...
  echo dax0.0 > /sys/bus/dax/drivers/kmem/unbind

Note: if unbind is done without offlining memory beforehand, it won't be
possible to do dax0.0 hotremove, and dax's memory is going to be part of
System RAM until reboot.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Pavel Tatashin <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Cc: James Morris <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Fengguang Wu <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Yaowei Bai <[email protected]>
Cc: Takashi Iwai <[email protected]>
Cc: Jérôme Glisse <[email protected]>
Cc: Dave Hansen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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