ethtool: tunnels: check the return value of nla_nest_start()

ethtool: tunnels: check the return value of nla_nest_start()

Check the return value of nla_nest_start(). When starting the entry
level nested attributes, if the tailroom of socket buffer is
insufficient to store the attribute header and payload, the return value
will be NULL.

There is, however, no real bug here since if the skb is full
nla_put_be16() will fail as well and we'll error out.

Signed-off-by: Li Zhong <floridsleeves@gmail.com>
Link: https://lore.kernel.org/r/20220921181716.1629541-1-floridsleeves@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>