tun: There is no longer any need to deny changing network namespaces

With the awkward case between free_netdev and dev_chr_close fixed
there is no longer any need to limit tun and tap devices to the
network namespace they were created in.  So remove the
NETIF_F_NETNS_LOCAL flag on the network device.

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