mm, thp: propagation of conditional compilation in khugepaged.c

Commit b46e756f5e47 ("thp: extract khugepaged from mm/huge_memory.c")
moved code from huge_memory.c to khugepaged.c.  Some of this code should
be compiled only when CONFIG_SYSFS is enabled but the condition around
this code was not moved into khugepaged.c.

The result is a compilation error when CONFIG_SYSFS is disabled:

  mm/built-in.o: In function `khugepaged_defrag_store': khugepaged.c:(.text+0x2d095): undefined reference to `single_hugepage_flag_store'
  mm/built-in.o: In function `khugepaged_defrag_show': khugepaged.c:(.text+0x2d0ab): undefined reference to `single_hugepage_flag_show'

This commit adds the #ifdef CONFIG_SYSFS around the code related to
sysfs.

Link: http://lkml.kernel.org/r/20161114203448.24197-1-jeremy.lefaure@lse.epita.fr
Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/khugepaged.c

@@ -103,6 +103,7 @@ static struct khugepaged_scan khugepaged_scan = {
 	.mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
 };
 
+#ifdef CONFIG_SYSFS
 static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,
 					 struct kobj_attribute *attr,
 					 char *buf)
@@ -295,6 +296,7 @@ struct attribute_group khugepaged_attr_group = {
 	.attrs = khugepaged_attr,
 	.name = "khugepaged",
 };
+#endif /* CONFIG_SYSFS */
 
 #define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB)