net: clear completed Buffers to permit earlier collection

Fixes #45163

Change-Id: Ie034145e3818930bb19371d73ec6960cbdc55aa7
Reviewed-on: https://go-review.googlesource.com/c/go/+/303829
Run-TryBot: Baokun Lee <[email protected]>
Trust: Baokun Lee <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>

Author: oiooj

src/net/net.go

@@ -733,6 +733,7 @@ func (v *Buffers) consume(n int64) {
 			return
 		}
 		n -= ln0
+		(*v)[0] = nil
 		*v = (*v)[1:]
 	}
 }