net: sched: act_tunnel_key: fix NULL pointer dereference during init

[ Upstream commit a3df633a3c92bb96b06552c3f828d7c267774379 ]

Metadata pointer is only initialized for action TCA_TUNNEL_KEY_ACT_SET, but
it is unconditionally dereferenced in tunnel_key_init() error handler.
Verify that metadata pointer is not NULL before dereferencing it in
tunnel_key_init error handling code.

Fixes: ee28bb56 ("net/sched: fix memory leak in act_tunnel_key_init()")
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>