connection: use current_cred() for sender policy checks

connection: use current_cred() for sender policy checks

This changes the policy-decisions for sender-initiated actions to use
current_cred() instead of conn->cred. This means, policy decisions for
ioctls will use the current credentials, instead of the credentials during
open().

On the other hand, receiver credentials are now bound to the pool used for
retrieval of the messages. Therefore, broadcasts and implicit policies are
checked against the file credentials of the target.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>