tun: Move read_wait into tun_file

The poll interface requires that the waitqueue exist while the struct
file is open.  In the rare case when a tun device disappears before
the tun file closes we fail to provide this property, so move
read_wait.

This is safe now that tun_net_xmit is atomic with tun_detach.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>