net: dsa: don't leak tagger-owned storage on switch driver unbind

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: don't leak tagger-owned storage on switch driver unbind

In the initial commit dc452a471dba ("net: dsa: introduce tagger-owned
storage for private and shared data"), we had a call to
tag_ops->disconnect(dst) issued from dsa_tree_free(), which is called at
tree teardown time.

There were problems with connecting to a switch tree as a whole, so this
got reworked to connecting to individual switches within the tree. In
this process, tag_ops->disconnect(ds) was made to be called only from
switch.c (cross-chip notifiers emitted as a result of dynamic tag proto
changes), but the normal driver teardown code path wasn't replaced with
anything.

Solve this problem by adding a function that does the opposite of
dsa_switch_setup_tag_protocol(), which is called from the equivalent
spot in dsa_switch_teardown(). The positioning here also ensures that we
won't have any use-after-free in tagging protocol (*rcv) ops, since the
teardown sequence is as follows:

dsa_tree_teardown
-> dsa_tree_teardown_master
-> dsa_master_teardown
-> unsets master->dsa_ptr, making no further packets match the
ETH_P_XDSA packet type handler
-> dsa_tree_teardown_ports
-> dsa_port_teardown
-> dsa_slave_destroy
-> unregisters DSA net devices, there is even a synchronize_net()
in unregister_netdevice_many()
-> dsa_tree_teardown_switches
-> dsa_switch_teardown
-> dsa_switch_teardown_tag_protocol
-> finally frees the tagger-owned storage