mount_setattr.2: EXAMPLES: use -1 rather than -EBADF

From email with Christian Braner:

> [1]: In this code "source" is expected to be absolute. If it's not
>      absolute we should fail. This can be achieved by passing -1/-EBADF,
>      afaict.
D'oh! Okay. I hadn't considered that use case for an invalid dirfd.
(And now I've done some adjustments to openat(2),which contains a
rationale for the *at() functions.)

So, now I understand your purpose, but still the code is obscure,
since

* You use a magic value (-EBADF) rather than (say) -1.
* There's no explanation (comment about) of the fact that you want
  to prevent relative pathnames.

So, I've changed the code to use -1, not -EBADF, and I've added some
comments to explain that the intent is to prevent relative pathnames.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/mount_setattr.2

@@ -961,7 +961,10 @@ main(int argc, char *argv[])
     const char *source = argv[optind];
     const char *target = argv[optind + 1];
 
-    int fd_tree = open_tree(\-EBADF, source,
+    /* In the following, \-1 as the \(aqdirfd\(aq argument ensures that
+       open_tree() fails if \(aqsource\(aq is not an absolute pathname. */
+
+    int fd_tree = open_tree(\-1, source,
                        OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC |
                        AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0));
     if (fd_tree == \-1)
@@ -980,7 +983,10 @@ main(int argc, char *argv[])
 
     close(fd_userns);
 
-    ret = move_mount(fd_tree, "", \-EBADF, target,
+    /* In the following, \-1 as the \(aqto_dirfd\(aq argument ensures that
+       open_tree() fails if \(aqtarget\(aq is not an absolute pathname. */
+
+    ret = move_mount(fd_tree, "", \-1, target,
                      MOVE_MOUNT_F_EMPTY_PATH);
     if (ret == \-1)
         exit_log("%m \- Failed to attach mount to %s\en", target);