unix: add Msghdr.SetIovlen

Fixes golang/go#34164

Change-Id: I77e634fafe8e573d5d2c65ddae9b0771c9f3e88f
GitHub-Last-Rev: dd25f06f860477c23992ceadaa3510bca983aa52
GitHub-Pull-Request: golang/sys#42
Reviewed-on: https://go-review.googlesource.com/c/sys/+/194077
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
diff --git a/unix/syscall_linux_mips64x.go b/unix/syscall_linux_mips64x.go
index b3b21ec..7562fe9 100644
--- a/unix/syscall_linux_mips64x.go
+++ b/unix/syscall_linux_mips64x.go
@@ -208,6 +208,10 @@
 	msghdr.Controllen = uint64(length)
 }
 
+func (msghdr *Msghdr) SetIovlen(length int) {
+	msghdr.Iovlen = uint64(length)
+}
+
 func (cmsg *Cmsghdr) SetLen(length int) {
 	cmsg.Len = uint64(length)
 }