KEYS: Fix race between key destruction and finding a keyring by name

KEYS: Fix race between key destruction and finding a keyring by name

There appears to be a race between:

 (1) key_gc_unused_keys() which frees key->security and then calls
     keyring_destroy() to unlink the name from the name list

 (2) find_keyring_by_name() which calls key_permission(), thus accessing
     key->security, on a key before checking to see whether the key usage is 0
     (ie. the key is dead and might be cleaned up).

Fix this by calling ->destroy() before cleaning up the core key data -
including key->security.

Reported-by: Petr Matousek <pmatouse@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Orabug: 22373388
(cherry picked from mainline commit 94c4554ba07adbdde396748ee7ae01e86cf2d8d7)
pre-requisite patch for mainline f05819df10d7b09f6d1eb6f8534a8f68e5a4fe61
Acked-by: Chuck Anderson <chuck.anderson@oracle.com>