MEDIUM: raw-sock: Specifiy amount of data to send via snd_pipe callback

MEDIUM: raw-sock: Specifiy amount of data to send via snd_pipe callback

When data were sent using the kernel splicing, we tried to send all data
with no restriction. Most of time it is valid. However, because the payload
representation may differ between the producer and the consumer, it is
important to be able to specify how must data to send via the splicing.

Of course, for performance reason, it is important to maximize amount of
data send via splicing at each call. However, on edge-cases, this now can be
limited.