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>