ktls: Improve handling of the bind_threads tunable a bit

- Only check for empty domains if we actually tried to configure domain
  affinity in the first place.  Otherwise setting bind_threads=1 will
  always cause the sysctl value to be reported as zero.  This is
  harmless since the threads end up being bound, but it's confusing.
- Try to improve the sysctl description a bit.

Reviewed by:	gallatin, jhb
Submitted by:	Klara, Inc.
Sponsored by:	Ampere Computing
Differential Revision:	https://reviews.freebsd.org/D28161

sys/kern/uipc_ktls.c

@@ -113,7 +113,7 @@ static int ktls_bind_threads;
 #endif
 SYSCTL_INT(_kern_ipc_tls, OID_AUTO, bind_threads, CTLFLAG_RDTUN,
     &ktls_bind_threads, 0,
-    "Bind crypto threads to cores or domains at boot");
+    "Bind crypto threads to cores (1) or cores and domains (2) at boot");
 
 static u_int ktls_maxlen = 16384;
 SYSCTL_UINT(_kern_ipc_tls, OID_AUTO, maxlen, CTLFLAG_RWTUN,
@@ -435,10 +435,12 @@ ktls_init(void *dummy __unused)
 	 * If we somehow have an empty domain, fall back to choosing
 	 * among all KTLS threads.
 	 */
-	for (i = 0; i < vm_ndomains; i++) {
-		if (ktls_domains[i].count == 0) {
-			ktls_bind_threads = 0;
-			break;
+	if (ktls_bind_threads > 1) {
+		for (i = 0; i < vm_ndomains; i++) {
+			if (ktls_domains[i].count == 0) {
+				ktls_bind_threads = 1;
+				break;
+			}
 		}
 	}