mm: validate device_hotplug is held for memory hotplug

mm: validate device_hotplug is held for memory hotplug

mem_hotplug_begin() assumes that it can set mem_hotplug.active_writer
and run the hotplug process without racing another thread.  Validate
this assumption with a lockdep assertion.

Link: http://lkml.kernel.org/r/148693886229.16345.1770484669403334689.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <[email protected]>
Reported-by: Ben Hutchings <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Masayoshi Mizuma <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>