mm: extract do_pages_move() out of sys_move_pages()

To prepare the chunking, move the sys_move_pages() code that is used when
nodes!=NULL into do_pages_move().  And rename do_move_pages() into
do_move_page_to_node_array().

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>