apparmor: fix race condition in label replacement

label replacement can result in the need for locking on two separate
trees. Currently this is done by locking the tree to remove and then
the tree to add to.

For compound labels the race can result in the old label proxy
pointing to the the new label that lost the race and that was not
inserted in to the new tree. This does not break mediation, but it does
result in a task that will not update its profile correctly on future
mediation, and that will leak its refcount due to a circular reference
in its proxy, resulting in a memory leak.

Signed-off-by: John Johansen <john.johansen@canonical.com>

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