tuntap: set SOCK_ZEROCOPY flag during open

[ Upstream commit 19a6afb2 ]

Commit 54f968d6
(tuntap: move socket to tun_file) forgets to set SOCK_ZEROCOPY flag, which will
prevent vhost_net from doing zercopy w/ tap. This patch fixes this by setting
it during file open.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>