tty: Don't forget to unblock tty peers

When testing for queued data present on
peers we setup non-blocking mode over them
to obtain -EAGAIN if there is no data
at all, but I forget to restore former
mode on return, ie blocking read.

Reported-by: Andrey Vagin <avagin@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>