Add size parameter to nanos6_tracepoint_c

nanos6_tracepoint_c needs to check if there is enough space in the
buffer before returning the buffer pointer to the user. If there is not,
it must flush the buffer to the storage device first. Also, after
flushing, it needs to write the nanos6:ctf_flush event. Both flushing
and writing the event can be done in the nanos6_tracepoint_c