thunderbolt: Fix NULL dereference in tb_remove_work()

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

thunderbolt: Fix NULL dereference in tb_remove_work()

There is a slight race between tb_remove_work() and tb_domain_remove()
which leads to dereferencing a NULL tb->root_switch pointer inside
tb_free_unplugged_xdomains():

Thread A Thread B

tb_remove_work()
tb_domain_remove()
mutex_lock(&tb->lock)
tb_stop()
/* doesn't cancel a running callback */
cancel_delayed_work(&tcm->remove_work)
...
tb_switch_remove(tb->root_switch)
tb->root_switch = NULL
mutex_unlock(&tb->lock)
mutex_lock(&tb->lock)
...
/* without checking ->root_switch */
tb_free_unplugged_xdomains(tb->root_switch)
mutex_unlock(&tb->lock)

Commit a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without
holding tb->lock") doesn't seem right to move tb_free_unplugged_xdomains()
out of the &tb->lock section and the check for tb->root_switch, in
particular. It states:

For this reason separate removing the XDomain from the topology data
structures (where we need the lock) from unregistering the device from
the bus (where remove callbacks of the drivers are being called).

tb_free_unplugged_xdomains() belongs to the former group of functions
requiring the lock. And it also calls tb_xdomain_remove() which should
only be called with &tb->lock held.

Found by Linux Verification Center (linuxtesting.org) with Svace static
analysis tool.