mm/sparse: more checks on mem_section number

__section_nr() was implemented to retrieve the corresponding memory
section number according to its descriptor.  It's possible that the
specified memory section descriptor doesn't exist in the global array.  So
add more checking on that and report an error for a wrong case.
Signed-off-by: NGavin Shan <shangw@linux.vnet.ibm.com>
Acked-by: NDavid Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -130,6 +130,8 @@ int __section_nr(struct mem_section* ms)
 		     break;
 	}
 
+	VM_BUG_ON(root_nr == NR_SECTION_ROOTS);
+
 	return (root_nr * SECTIONS_PER_ROOT) + (ms - root);
 }