MINOR: mux-h2: always use h2_send() in h2_done_ff(), not h2_process()

MINOR: mux-h2: always use h2_send() in h2_done_ff(), not h2_process()

By calling h2_process(), the code would theoretically make it possible
for a synchronous ->wake() call to provoke an indirect call to h2_snd_buf()
while we're in h2_done_ff(), which could be quite bad. The current
conditions do not permit it right now but this could easily break by
accident. Better use h2_send() and wake the task up if needed. Precise
performance tests showed no change.