Calculate context length outside of retry loop

Calculate context length outside of retry loop

Allow context length calculation to have side-effects (e.g. page faults)
which trigger event tracing by moving the calculation outside of the
buffer space reservation retry loop.

This also paves the way to have dynamically sized contexts, which
would expect to put their size of the internal stack. Note that the
context length calculation is performed *after* the event payload field
length calculation, so the stack needs to be used accordingly.

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