net: sched: do not allocate a tracker in tcf_exts_init()

While struct tcf_exts has a net pointer, it is not refcounted
until tcf_exts_get_net() is called.

Fixes: dbdcda63

 ("net: sched: add netns refcount tracker to struct tcf_exts")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Link: https://lore.kernel.org/r/20220110094750.236478-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>