tun: move internal flag defines out of uapi

TUN_ flags are internal and never exposed
to userspace. Any application using it is almost
certainly buggy.

Move them out to tun.c.

Note: we remove these completely in follow-up patches,
this code movement is split out for ease of review.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --cc drivers/net/tun.c
Simple merge

diff --cc include/uapi/linux/if_tun.h
Simple merge