apparmor: fix reference count leak in aa_pivotroot()

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

apparmor: fix reference count leak in aa_pivotroot()

The aa_pivotroot() function has a reference counting bug in a specific

path. When aa_replace_current_label() returns on success, the function

forgets to decrement the reference count of “target”, which is

increased earlier by build_pivotroot(), causing a reference leak.

Fix it by decreasing the refcount of “target” in that path.