BUG/MINOR: mux-quic: differentiate failure on qc_stream_desc alloc

BUG/MINOR: mux-quic: differentiate failure on qc_stream_desc alloc

qc_stream_buf_alloc() can fail for two reasons :
* limit of Tx buffer per connection reached
* allocation failure

The first case is properly treated. A flag QC_CF_CONN_FULL is set on the
connection to interrupt emission. It is cleared when a buffer became
available after in order ACK reception and the MUX tasklet is woken up.

The allocation failure was handled with the same mechanism which in this
case is not appropriate and could lead to a connection transfer freeze.
Instead, prefer to close the connection with a QUIC internal error code.

To differentiate the two causes, qc_stream_buf_alloc() API was changed
to return the number of available buffers to the caller.

This must be backported up to 2.6.