When splicing page server data from UNIX socket we may get
error (EAGAIN) from splice if no data is available on the
socket yet. This is because the SPLICE_F_NONBLOCK flag is
checked by af_unix.c in the kernel to decide whether or
not to do blocking read.

This is not symmetrical with TCP sockets, which only check
for the socket's O_NONBLOCK flag for the same decicion.

Dropping the SPLICE_F_NONBLOCK flag is not possible too, as
otherwise we'll block on the pipe when trying to put data
into it. Even if part of the data fits into it kernel would
block anyway untill full buffer is in. And there will be
no read() from the pipe, as it should happen one step later
in the same task.

So to untie this, we need to wait for the data explicitly
with poll().
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>