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.

Fixes: 9578e327 ("keys: update key quotas in key_put()")
Reported-by:  <syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/673b6aec.050a0220.87769.004a.GAE@google.com/


Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by:  <syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>