Currently, we may set the tunnel option flag when the size of metadata
is zero.  For example, we set TUNNEL_GENEVE_OPT in the receive function
no matter the geneve option is present or not.  As this may result in
issues on the tunnel flags consumers, this patch fixes the issue.

Related discussion:
* https://lore.kernel.org/netdev/1604448694-19351-1-git-send-email-yihung.wei@gmail.com/T/#u

Fixes: 256c87c1 ("net: check tunnel option type in tunnel flags")
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Link: https://lore.kernel.org/r/1605053800-74072-1-git-send-email-yihung.wei@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>