CLEANUP: Use ist2(const void*, size_t) whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    struct ist i;
    expression p, l;
    @@

    - i.ptr = p;
    - i.len = l;
    + i = ist2(p, l);
diff --git a/src/dns.c b/src/dns.c
index 835975c..dcdc163 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -105,8 +105,7 @@
 			if (errno == EAGAIN) {
 				struct ist myist;
 
-				myist.ptr = buf;
-				myist.len = len;
+				myist = ist2(buf, len);
 				ret = ring_write(ns->dgram->ring_req, DNS_TCP_MSG_MAX_SIZE, NULL, 0, &myist, 1);
 				if (!ret) {
 					ns->counters->snd_error++;
@@ -125,8 +124,7 @@
 	else if (ns->stream) {
 		struct ist myist;
 
-		myist.ptr = buf;
-		myist.len = len;
+		myist = ist2(buf, len);
                 ret = ring_write(ns->stream->ring_req, DNS_TCP_MSG_MAX_SIZE, NULL, 0, &myist, 1);
 		if (!ret) {
 			ns->counters->snd_error++;
@@ -1123,8 +1121,7 @@
 
 		len = b_getblk(buf, dns_msg_trash, msg_len, ofs + cnt);
 
-		myist.ptr = dns_msg_trash;
-		myist.len = len;
+		myist = ist2(dns_msg_trash, len);
 
 		ads = NULL;
 		/* try to push request into active sess with free slot */