tools/libxs: Fix fcntl() invocation in set_cloexec()

set_cloexec() had a bit too much copy&pate from setnonblock(), and
insufficient testing on ancient versions of Linux...

As written (emulating ancient linux by undef'ing O_CLOEXEC), strace shows:

  open("/dev/xen/xenbus", O_RDWR)         = 3
  fcntl(3, F_GETFL)                       = 0x8002 (flags O_RDWR|O_LARGEFILE)
  fcntl(3, 0x8003 /* F_??? */, 0x7ffe4a771d90) = -1 EINVAL (Invalid argument)
  close(3)                                = 0

which is obviously nonsense.

Switch F_GETFL -> F_GETFD, and fix the second invocation to use F_SETFD.  With
this, strace is rather happer:

  open("/dev/xen/xenbus", O_RDWR)         = 3
  fcntl(3, F_GETFD)                       = 0
  fcntl(3, F_SETFD, FD_CLOEXEC)           = 0

Fixes: bf7c146470 ("tools/libxs: Fix CLOEXEC handling in get_dev()")
Reported-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

tools/libs/store/xs.c

@@ -182,12 +182,12 @@ static bool setnonblock(int fd, int nonblock) {
 
 static bool set_cloexec(int fd)
 {
-	int flags = fcntl(fd, F_GETFL);
+	int flags = fcntl(fd, F_GETFD);
 
 	if (flags < 0)
 		return false;
 
-	return fcntl(fd, flags | FD_CLOEXEC) >= 0;
+	return fcntl(fd, F_SETFD, flags | FD_CLOEXEC) >= 0;
 }
 
 static int pipe_cloexec(int fds[2])