unix: add OpenTree on linux

This syscall is not yet described in linux man pages
(https://github.com/mkerrisk/man-pages.git)
but it was attempts to do it (https://lwn.net/Articles/829496/)
This syscall appeared in linux kernel v5.2

Change-Id: If44aa74c4ddd200831011c6c4bf4bd4fee440785
Reviewed-on: https://go-review.googlesource.com/c/sys/+/397094
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/zsyscall_linux.go b/unix/zsyscall_linux.go
index 78d7452..ca65ac8 100644
--- a/unix/zsyscall_linux.go
+++ b/unix/zsyscall_linux.go
@@ -1215,6 +1215,22 @@
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func OpenTree(dfd int, fileName string, flags uint) (r int, err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(fileName)
+	if err != nil {
+		return
+	}
+	r0, _, e1 := Syscall(SYS_OPEN_TREE, uintptr(dfd), uintptr(unsafe.Pointer(_p0)), uintptr(flags))
+	r = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func PerfEventOpen(attr *PerfEventAttr, pid int, cpu int, groupFd int, flags int) (fd int, err error) {
 	r0, _, e1 := Syscall6(SYS_PERF_EVENT_OPEN, uintptr(unsafe.Pointer(attr)), uintptr(pid), uintptr(cpu), uintptr(groupFd), uintptr(flags), 0)
 	fd = int(r0)