mm/memory_hotplug.c: add a bounds check to __add_pages()

mm/memory_hotplug.c: add a bounds check to __add_pages()

On PowerPC, the address ranges allocated to OpenCAPI LPC memory are
allocated from firmware.  These address ranges may be higher than what
older kernels permit, as we increased the maximum permissable address in
commit 76ff386e271a ("powerpc/mm: Increase the max addressable memory to
2PB").  It is possible that the addressable range may change again in
the future.

In this scenario, we end up with a bogus section returned from
__section_nr (see the discussion on the thread "mm: Trigger bug on if a
section is not found in __section_nr").

Adding a check here means that we fail early and have an opportunity to
handle the error gracefully, rather than rumbling on and potentially
accessing an incorrect section.

Further discussion is also on the thread ("powerpc: Perform a bounds
check in arch_add_memory")
  http://lkml.kernel.org/r/20190827052047.31547-1-alastair@au1.ibm.com

Link: http://lkml.kernel.org/r/20191001004617.7536-2-alastair@au1.ibm.com
Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>