rxrpc: Downgrade the BUG() for unsupported token type in rxrpc_read()

If rxrpc_read() (which allows KEYCTL_READ to read a key), sees a token of a
type it doesn't recognise, it can BUG in a couple of places, which is
unnecessary as it can easily get back to userspace.

Fix this to print an error message instead.

Fixes: 99455153

 ("RxRPC: Parse security index 5 keys (Kerberos 5)")
Signed-off-by: David Howells <dhowells@redhat.com>