apparmor: don't audit files pointing to aa_null.dentry

apparmor: don't audit files pointing to aa_null.dentry

In
  commit 4a134723f9f1 ("apparmor: move check for aa_null file to cover all cases")
there was a change to not audit files pointing to
aa_null.dentry because they provide no value, but setting the error
variable instead of returning -EACCES was still causing them to be
audited.

Fixes: 4a134723f9f1 ("apparmor: move check for aa_null file to cover all cases")
Acked-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>