SUNRPC: fix sign error causing rpcsec_gss drops

If sd_max is unsigned, then sd_max - GSS_SEQ_WIN is a very large number
whenever sd_max is less than GSS_SEQ_WIN, and the comparison:

	seq_num <= sd->sd_max - GSS_SEQ_WIN

in gss_check_seq_num is pretty much always true, even when that's
clearly not what was intended.

This was causing pynfs to hang when using krb5, because pynfs uses zero
as the initial gss sequence number.  That's perfectly legal, but this
logic error causes knfsd to drop the rpc in that case.  Out-of-order
sequence IDs in the first GSS_SEQ_WIN (128) calls will also cause this.

Fixes: 10b9d99 ("SUNRPC: Augment server-side rpcgss tracepoints")
Cc: [email protected]
Signed-off-by: J. Bruce Fields <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

Author: J. Bruce Fields

net/sunrpc/auth_gss/svcauth_gss.c

@@ -645,7 +645,7 @@ static bool gss_check_seq_num(const struct svc_rqst *rqstp, struct rsc *rsci,
 		}
 		__set_bit(seq_num % GSS_SEQ_WIN, sd->sd_win);
 		goto ok;
-	} else if (seq_num <= sd->sd_max - GSS_SEQ_WIN) {
+	} else if (seq_num + GSS_SEQ_WIN <= sd->sd_max) {
 		goto toolow;
 	}
 	if (__test_and_set_bit(seq_num % GSS_SEQ_WIN, sd->sd_win))