follow_automount(): get rid of dead^Wstillborn code

1) no instances of ->d_automount() have ever made use of the "return
ERR_PTR(-EISDIR) if you don't feel like mounting anything" - that's
a rudiment of plans that got superseded before the thing went into
the tree.  Despite the comment in follow_automount(), autofs has
never done that.

2) if there's no ->d_automount() in dentry_operations, filesystems
should not set DCACHE_NEED_AUTOMOUNT in the first place.  None have
ever done so...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/namespace.c b/fs/namespace.c
index 777c311..7439803 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2824,9 +2824,16 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
 int finish_automount(struct vfsmount *m, struct path *path)
 {
 	struct dentry *dentry = path->dentry;
-	struct mount *mnt = real_mount(m);
 	struct mountpoint *mp;
+	struct mount *mnt;
 	int err;
+
+	if (!m)
+		return 0;
+	if (IS_ERR(m))
+		return PTR_ERR(m);
+
+	mnt = real_mount(m);
 	/* The new mount record should have at least 2 refs to prevent it being
 	 * expired before we get a chance to add it
 	 */