BUG/MINOR: stream_interface: don't call chk_snd() on polled events

BUG/MINOR: stream_interface: don't call chk_snd() on polled events

As explained in previous patch, we incorrectly call chk_snd() when
performing a read even if the write event is already subscribed to
poll(). This is counter-productive because we're almost sure to get
an EAGAIN.

A quick test shows that this fix halves the number of failed splice()
calls without adding any extra work on other syscalls.

This could have been tagged as an improvement, but since this behaviour
made the analysis of previous bug more complex, it still qualifies as
a fix.