iov_iter stuff, part 2, rebased

* more new_sync_{read,write}() speedups - ITER_UBUF introduction
* ITER_PIPE cleanups
* unification of iov_iter_get_pages/iov_iter_get_pages_alloc and
  switching them to advancing semantics
* making ITER_PIPE take high-order pages without splitting them
* handling copy_page_from_iter() for high-order pages properly

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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