gss_krb5: document that we ignore sequence number

A couple times recently somebody has noticed that we're ignoring a
sequence number here and wondered whether there's a bug.

In fact, there's not.  Thanks to Andy Adamson for pointing out a useful
explanation in rfc 2203.  Add comments citing that rfc, and remove
"seqnum" to prevent static checkers complaining about unused variables.
Reported-by: NAndi Kleen <andi@firstfloor.org>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/gss_krb5_unseal.c

@@ -150,7 +150,6 @@ gss_verify_mic_v2(struct krb5_ctx *ctx,
 	struct xdr_netobj cksumobj = {.len = sizeof(cksumdata),
 				      .data = cksumdata};
 	s32 now;
-	u64 seqnum;
 	u8 *ptr = read_token->data;
 	u8 *cksumkey;
 	u8 flags;
@@ -197,9 +196,10 @@ gss_verify_mic_v2(struct krb5_ctx *ctx,
 	if (now > ctx->endtime)
 		return GSS_S_CONTEXT_EXPIRED;
 
-	/* do sequencing checks */
-
-	seqnum = be64_to_cpup((__be64 *)ptr + 8);
+	/*
+	 * NOTE: the sequence number at ptr + 8 is skipped, rpcsec_gss
+	 * doesn't want it checked; see page 6 of rfc 2203.
+	 */
 
 	return GSS_S_COMPLETE;
 }

net/sunrpc/auth_gss/gss_krb5_wrap.c

@@ -489,7 +489,6 @@ static u32
 gss_unwrap_kerberos_v2(struct krb5_ctx *kctx, int offset, struct xdr_buf *buf)
 {
 	s32		now;
-	u64		seqnum;
 	u8		*ptr;
 	u8		flags = 0x00;
 	u16		ec, rrc;
@@ -525,7 +524,10 @@ gss_unwrap_kerberos_v2(struct krb5_ctx *kctx, int offset, struct xdr_buf *buf)
 	ec = be16_to_cpup((__be16 *)(ptr + 4));
 	rrc = be16_to_cpup((__be16 *)(ptr + 6));
 
-	seqnum = be64_to_cpup((__be64 *)(ptr + 8));
+	/*
+	 * NOTE: the sequence number at ptr + 8 is skipped, rpcsec_gss
+	 * doesn't want it checked; see page 6 of rfc 2203.
+	 */
 
 	if (rrc != 0)
 		rotate_left(offset + 16, buf, rrc);