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: Andi Kleen <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>
diff --git a/net/sunrpc/auth_gss/gss_krb5_unseal.c b/net/sunrpc/auth_gss/gss_krb5_unseal.c
index 6cd930f..6c981dd 100644
--- a/net/sunrpc/auth_gss/gss_krb5_unseal.c
+++ b/net/sunrpc/auth_gss/gss_krb5_unseal.c
@@ -150,7 +150,6 @@
 	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 @@
 	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;
 }