consumerd: move rotation logic to domain-agnostic read path

consumerd: move rotation logic to domain-agnostic read path

The "rotation ready" logic is duplicated in both user space and kernel
specializations of the read subbuffer functions.

It is moved to the domain-agnostic caller where it is needed only
once. This makes it easier to implement a follow-up fix and reduces
code duplication.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iae952a2cd52fa458cec956ae219492557e4adf79