mm/sparse.c: introduce new function fill_subsection_map()

Patch series "mm/hotplug: Only use subsection map for VMEMMAP", v4.

Memory sub-section hotplug was added to fix the issue that nvdimm could be
mapped at non-section aligned starting address.  A subsection map is added
into struct mem_section_usage to implement it.

However, config ZONE_DEVICE depends on SPARSEMEM_VMEMMAP.  It means
subsection map only makes sense when SPARSEMEM_VMEMMAP enabled.  For the
classic sparse, subsection map is meaningless and confusing.

About the classic sparse which doesn't support subsection hotplug, Dan
said it's more because the effort and maintenance burden outweighs the
benefit.  Besides, the current 64 bit ARCHes all enable
SPARSEMEM_VMEMMAP_ENABLE by default.

This patch (of 5):

Factor out the code that fills the subsection map from section_activate()
into fill_subsection_map(), this makes section_activate() cleaner and
easier to follow.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Link: http://lkml.kernel.org/r/20200312124414.439-2-bhe@redhat.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>