thp: extract khugepaged from mm/huge_memory.c

khugepaged implementation grew to the point when it deserve separate
file in source.

Let's move it to mm/khugepaged.c.

Link: http://lkml.kernel.org/r/1466021202-61880-32-git-send-email-kirill.shutemov@linux.intel.com


Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>