Calculate context length outside of retry loop

Allow context length calculation to have side-effects 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 in lttng-ust,
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.

Currently, the only dynamically sized contexts we have are provided by
Java integration, which keeps its own stack.

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