BUG/MINOR: stream_interface: don't loop over ->snd_buf()

BUG/MINOR: stream_interface: don't loop over ->snd_buf()

It is stupid to loop over ->snd_buf() because the snd_buf() itself already
loops and stops when system buffers are full. But looping again onto it,
we lose the information of the full buffers and perform one useless syscall.

Furthermore, this causes issues when dealing with large uploads while waiting
for a connection to establish, as it can report a server reject of some data
as a connection abort, which is wrong.

1.4 does not have this issue as it loops maximum twice (once for each buffer
half) and exists as soon as system buffers are full. So no backport is needed.