make __section_nr() more efficient

When CONFIG_SPARSEMEM_EXTREME is disabled, __section_nr can get the
section number with a subtraction directly.

Link: http://lkml.kernel.org/r/1468988310-11560-1-git-send-email-zhouchengming1@huawei.com
Signed-off-by: Zhou Chengming <zhouchengming1@huawei.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Li Bin <huawei.libin@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -100,11 +100,7 @@ static inline int sparse_index_init(unsigned long section_nr, int nid)
 }
 #endif
 
-/*
- * Although written for the SPARSEMEM_EXTREME case, this happens
- * to also work for the flat array case because
- * NR_SECTION_ROOTS==NR_MEM_SECTIONS.
- */
+#ifdef CONFIG_SPARSEMEM_EXTREME
 int __section_nr(struct mem_section* ms)
 {
 	unsigned long root_nr;
@@ -123,6 +119,12 @@ int __section_nr(struct mem_section* ms)
 
 	return (root_nr * SECTIONS_PER_ROOT) + (ms - root);
 }
+#else
+int __section_nr(struct mem_section* ms)
+{
+	return (int)(ms - mem_section[0]);
+}
+#endif
 
 /*
  * During early boot, before section_mem_map is used for an actual