Refactor struct lttng_ust_lib_ring_buffer_ctx

- Remove padding entirely. Will be replaced by checks against
  @struct_size.
- Rename @ctx_len to @struct_size, to align with the rest of the other
  structures.
- Remove priv vs priv2 private data fields. "priv" held a pointer only
  useful for legacy ABI. Set priv2 to the lttng_ctx used in the current
  ABI instead.
- Remove checks for @ctx_len only useful to support old ABI.
- Remove fallback code for old ABI.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I1273be9615846304266206769700e799bde4e51f