ring buffer context: cpu number becomes an output of reserve

In order to facilitate eventual integration of a ring buffer scheme
based on Restartable Sequences (sys_rseq), change the ownership of the
ring buffer context "cpu" field so it is now populated by the ring
buffer reserve operation. This means a rseq-based reserve could retry
on a new current cpu after a rseq-cmpxchg fails to reserve and then a
migration occurs.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: If0c0689446975085b5e22b14aef6a15f12f8ff9f