udp: downgrade the per-batch sendmmsg failure log to debug

udp/udp_linux_writebatch.go

@@ -3,6 +3,7 @@
 package udp
 
 import (
+	"context"
 	"encoding/binary"
 	"errors"
 	"fmt"
@@ -324,7 +325,8 @@ sendChunks:
 			if ecns != nil {
 				ecn0 = ecns[baseI]
 			}
-			w.l.Warn("sendmmsg had problem",
+			if w.l.Enabled(context.Background(), slog.LevelDebug) {
+				w.l.Debug("sendmmsg had problem",
 					"sent", sent, "err", serr,
 					"entries", entry,
 					"entry0_runLen", runLen0,
@@ -337,6 +339,7 @@ sendChunks:
 					"isV4", w.isV4,
 					"gso", w.gsoSupported,
 				)
+			}
 			for k := baseI; k < i; k++ {
 				if werr := sendto(w.fd, bufs[k], addrs[k], w.isV4); werr == nil {
 					written++