Move channel context to private structures

Move channel context to private structures

The list of contexts active for a channel can be moved to a private
structure by keeping the temporary pointer to the RCU dereferenced
context in the ring buffer client's internal context rather than within
the context shared between the probe and the ring buffer client.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Iaf9491255d1a0940450dea613ac6e5bc0705506d