Replace magic for trusting the secondary keyring with #define

commit 817aef260037f33ee0f44c17fe341323d3aebd6d upstream.

Replace the use of a magic number that indicates that verify_*_signature()
should use the secondary keyring with a symbol.

Signed-off-by: Yannik Sembritzki <yannik@sembritzki.me>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: keyrings@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>