unix: implement Vmsplice, add SPLICE_F_* constants

Fixes golang/go#19125

Change-Id: I822b6f260f980178da29a552c6170cb261c8e254
Reviewed-on: https://go-review.googlesource.com/37152
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -1118,6 +1118,25 @@ func Munmap(b []byte) (err error) {
 //sys	Mlockall(flags int) (err error)
 //sys	Munlockall() (err error)
 
+// Vmsplice splices user pages from a slice of Iovecs into a pipe specified by fd,
+// using the specified flags.
+func Vmsplice(fd int, iovs []Iovec, flags int) (int, error) {
+	n, _, errno := Syscall6(
+		SYS_VMSPLICE,
+		uintptr(fd),
+		uintptr(unsafe.Pointer(&iovs[0])),
+		uintptr(len(iovs)),
+		uintptr(flags),
+		0,
+		0,
+	)
+	if errno != 0 {
+		return 0, syscall.Errno(errno)
+	}
+
+	return int(n), nil
+}
+
 /*
  * Unimplemented
  */
@@ -1245,7 +1264,6 @@ func Munmap(b []byte) (err error) {
 // Utimensat
 // Vfork
 // Vhangup
-// Vmsplice
 // Vserver
 // Waitid
 // _Sysctl