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.