keys: Fix UAF in key_put()

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

keys: Fix UAF in key_put()

Once a key's reference count has been reduced to 0, the garbage collector

thread may destroy it at any time and so key_put() is not allowed to touch

the key after that point. The most key_put() is normally allowed to do is

to touch key_gc_work as that's a static global variable.

However, in an effort to speed up the reclamation of quota, this is now

done in key_put() once the key's usage is reduced to 0 - but now the code

is looking at the key after the deadline, which is forbidden.

Fix this by using a flag to indicate that a key can be gc'd now rather than

looking at the key's refcount in the garbage collector.