LU-13814 osc: Move osc_page members to osc_async_page

LU-13814 osc: Move osc_page members to osc_async_page

We're going to start using osc_async_page separately from
osc_page, since it's used for both DIO and buffered, but
osc_page is not.

This moves all of the members which are needed for DIO.

This commit deliberately ignores some packing related
issues which will be handled *much* later, since the names
and members of these structs will keep changing.

Test-Parameters: forjanitoronly
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I994af9563142201a2c7193735ac02568735bd8d5