ip_gre: clear feature flags when incompatible o_flags are set

Commit dd9d598c ("ip_gre: add the support for i/o_flags update via
netlink") added the ability to change o_flags, but missed that the
GSO/LLTX features are disabled by default, and only enabled some gre
features are unused. Thus we also need to disable the GSO/LLTX features
on the device when the TUNNEL_SEQ or TUNNEL_CSUM flags are set.

These two examples should result in the same features being set:

    ip link add gre_none type gre local 192.168.0.10 remote 192.168.0.20 ttl 255 key 0

    ip link set gre_none type gre seq
    ip link add gre_seq type gre local 192.168.0.10 remote 192.168.0.20 ttl 255 key 1 seq

Fixes: dd9d598c

 ("ip_gre: add the support for i/o_flags update via netlink")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>