KEYS: request_key() should return -ENOKEY if the constructed key is negative

request_key() should return -ENOKEY if the key it constructs has been
negatively instantiated.

Without this, request_key() can return an unusable key to its caller,
and if the caller then does key_validate() that won't catch the problem.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>