rbd: move from raw pages to bvec data descriptors

In preparation for rbd "fancy" striping which requires bio_vec arrays,
wire up BVECS data type and kill off PAGES data type.  There is nothing
wrong with using page vectors for copyup requests -- it's just less
iterator boilerplate code to write for the new striping framework.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>