KEYS: load key flags and expiry time atomically in proc_keys_show()

In proc_keys_show(), the key semaphore is not held, so the key ->flags
and ->expiry can be changed concurrently.  We therefore should read them
atomically just once.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>