page server: Don't hide error from flush request

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
tcp: don't fail if send eats only a part of data

For stream sockets it's possiable and we should not fail in this case.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>