net: split __zerocopy_sg_from_iter()

Split a function out of __zerocopy_sg_from_iter() that only cares about
the traditional path with refcounted pages and doesn't need to know
about ->sg_from_iter. A preparation patch, we'll improve on the function
later.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>