Revert "net: sched: fw: don't set arg->stop in fw_walk() when empty"

This reverts commit 31a99848 ("net: sched: fw: don't set arg->stop in
fw_walk() when empty")

Cls API function tcf_proto_is_empty() was changed in commit
6676d5e4 ("net: sched: set dedicated tcf_walker flag when tp is empty")
to no longer depend on arg->stop to determine that classifier instance is
empty. Instead, it adds dedicated arg->nonempty field, which makes the fix
in fw classifier no longer necessary.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>