lib/scatterlist: use page iterator in the mapping iterator

lib/scatterlist: use page iterator in the mapping iterator

For better code reuse use the newly added page iterator to iterate through
the pages.  The offset, length within the page is still calculated by the
mapping iterator as well as the actual mapping.  Idea from Tejun Heo.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>