afs: Fix debugging statements with %px to be %p

Fix a couple of %px to be %p in debugging statements.

Fixes: e49c7b2f ("afs: Build an abstraction around an "operation" concept")
Fixes: 8a070a96 ("afs: Detect cell aliases 1 - Cells with root volumes")
Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>