mm/sparse.c: move subsection_map related functions together

No functional change.

[bhe@redhat.com: move functions into CONFIG_MEMORY_HOTPLUG ifdeffery scope]
  Link: http://lkml.kernel.org/r/20200316045804.GC3486@MiWiFi-R3L-srv


Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Link: http://lkml.kernel.org/r/20200312124414.439-6-bhe@redhat.com


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