LU-13814 clio: add cl_dio_pages_init

LU-13814 clio: add cl_dio_pages_init

Just like the cl_page it's replacing, the cl_dio_pages
struct needs various pieces of information from the
different layers of the cl_object in order to do the IO.

This means we need a cl_dio_pages init, analogous to
cl_page_alloc and coo_pages_init.

Note this does not implement coo_pages_init for any layers,
it just moves parts of the existing init code and lays out
the function structure.

Test-Parameters: fortestonly
Test-Parameters: forjanitoronly
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I1fcf407b16d4077d94c7ba5afbc63bdd3fb3dfb4