KEYS: Make request key instantiate the per-user keyrings

Make request_key() instantiate the per-user keyrings so that it doesn't oops
if it needs to get hold of the user session keyring because there isn't a
session keyring in place.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Steve French <smfrench@gmail.com>
Tested-by: Rutger Nijlunsing <rutger.nijlunsing@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>