BUG/MEDIUM: mux-h2: always return already allocated buffers in h2_get_buf()

BUG/MEDIUM: mux-h2: always return already allocated buffers in h2_get_buf()

h2_get_buf() is used to get the demux buffer, the mux buffer ring's tail
and the streams' rxbufs. It refrains from calling b_alloc() when the
connection is already in the buffer wait queue, but it does so even when
the requested buffer is already allocated, in which case it returns NULL.
The consequence is that as soon as an H2 connection waits for any buffer
(e.g. a stream's rxbuf), all other users consider that their already
allocated buffer is missing: h2_recv() stops reading and sets
H2_CF_DEM_DALLOC although the demux buffer is allocated, frame emitters
set H2_CF_MUX_MALLOC although the mbuf has room, etc. The connection then
stops both receiving and sending until the buffer callback is called,
and these spurious flags confuse h2_buf_available() which only serves one
of them (see next fixes).

This can be observed by limiting the number of buffers with
tune.buffers.limit, holding them with incomplete HTTP/1 requests, then
sending an H2 request: once the stream's rxbuf allocation fails, the
connection reports a demux buffer allocation failure on the next recv
attempt (seen with debug traces).

Let's always return an already allocated buffer. This has been there
since the merge of the buffer allocators in 1.9 with commit 44e973f50
("MEDIUM: h2: use a single buffer allocator"), so this fix may be
backported to all supported versions.