apparmor: Fix an error code in verify_table_headers()

We accidentally return a positive EPROTO instead of a negative -EPROTO.
Since 71 is not an error pointer, that means it eventually results in an
Oops in the caller.

Fixes: d901d6a2 ("apparmor: dfa split verification of table headers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>