treewide: Add SPDX identifier to IETF ASN.1 modules

Per section 4.c. of the IETF Trust Legal Provisions, "Code Components"
in IETF Documents are licensed on the terms of the BSD-3-Clause license:

https://trustee.ietf.org/documents/trust-legal-provisions/tlp-5/

The term "Code Components" specifically includes ASN.1 modules:

https://trustee.ietf.org/documents/trust-legal-provisions/code-components-list-3/

Add an SPDX identifier as well as a copyright notice pursuant to section
6.d. of the Trust Legal Provisions to all ASN.1 modules in the tree
which are derived from IETF Documents.

Section 4.d. of the Trust Legal Provisions requests that each Code
Component identify the RFC from which it is taken, so link that RFC
in every ASN.1 module.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/asymmetric_keys/pkcs7.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 2009 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc5652#section-3
+
 PKCS7ContentInfo ::= SEQUENCE {
 	contentType	ContentType ({ pkcs7_check_content_type }),
 	content		[0] EXPLICIT SignedData OPTIONAL

crypto/asymmetric_keys/pkcs8.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 2010 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc5958#section-2
 --
 -- This is the unencrypted variant
 --

crypto/asymmetric_keys/x509.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 2008 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc5280#section-4
+
 Certificate ::= SEQUENCE {
 	tbsCertificate		TBSCertificate ({ x509_note_tbs_certificate }),
 	signatureAlgorithm	AlgorithmIdentifier,

crypto/asymmetric_keys/x509_akid.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 2008 IETF Trust and the persons identified as authors
+-- of the code
+--
 -- X.509 AuthorityKeyIdentifier
 -- rfc5280 section 4.2.1.1
 

crypto/rsaprivkey.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 2016 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc8017#appendix-A.1.2
+
 RsaPrivKey ::= SEQUENCE {
 	version		INTEGER,
 	n		INTEGER ({ rsa_get_n }),

crypto/rsapubkey.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 2016 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc8017#appendix-A.1.1
+
 RsaPubKey ::= SEQUENCE {
 	n INTEGER ({ rsa_get_n }),
 	e INTEGER ({ rsa_get_e })

fs/smb/server/ksmbd_spnego_negtokeninit.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 1998, 2000 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc2478#section-3.2.1
+-- https://www.rfc-editor.org/rfc/rfc2743#section-3.1
+
 GSSAPI ::=
 	[APPLICATION 0] IMPLICIT SEQUENCE {
 		thisMech

fs/smb/server/ksmbd_spnego_negtokentarg.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 1998 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc2478#section-3.2.1
+
 GSSAPI ::=
 	CHOICE {
 		negTokenInit

net/ipv4/netfilter/nf_nat_snmp_basic.asn1

+-- SPDX-License-Identifier: BSD-3-Clause
+--
+-- Copyright (C) 1990, 2002 IETF Trust and the persons identified as authors
+-- of the code
+--
+-- https://www.rfc-editor.org/rfc/rfc1157#section-4
+-- https://www.rfc-editor.org/rfc/rfc3416#section-3
+
 Message ::=
 	SEQUENCE {
 		version