OPTIM: stream_sock: reduce the amount of in-flight spliced data

OPTIM: stream_sock: reduce the amount of in-flight spliced data

First, it's a waste not to call chk_snd() when spliced data are available,
because the pipe can almost always be transferred into the outgoing socket
buffers. Starting from now, when we splice data in, we immediately try to
send them. This results in less pipes used, and possibly less kernel memory
in use at once.

Second, if a pipe cannot be transferred into the outgoing socket buffers,
it means this buffer is full. There's no point trying again then, as space
will almost never be available, resulting in a useless syscall returning
EAGAIN.