tun: fix build warnings

Stephen Rothwell reports a build warnings(powerpc ppc64_defconfig)

drivers/net/tun.c: In function 'tun_do_read.part.5':
/home/sfr/next/next/drivers/net/tun.c:1491:6: warning: 'err' may be
used uninitialized in this function [-Wmaybe-uninitialized]
   int err;

This is because tun_ring_recv() may return an uninitialized err, fix this.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>