fs/proc: fix potential unregister_sysctl_table hang

The unregister_sysctl_table() function hangs if all references to its
ctl_table_header structure are not dropped.

This can happen sometimes because of a leak in proc_sys_lookup():
proc_sys_lookup() gets a reference to the table via lookup_entry(), but
it does not release it when a subsequent call to sysctl_follow_link()
fails.

This patch fixes this leak by making sure the reference is always
dropped on return.

See also commit 076c3eed ("sysctl: Rewrite proc_sys_lookup
introducing find_entry and lookup_entry") which reorganized this code in
3.4.

Tested in Linux 3.4.4.
Signed-off-by: Francesco Ruggeri <fruggeri@aristanetworks.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/proc_sysctl.c

@@ -462,9 +462,6 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
 
 	err = ERR_PTR(-ENOMEM);
 	inode = proc_sys_make_inode(dir->i_sb, h ? h : head, p);
-	if (h)
-		sysctl_head_finish(h);
-
 	if (!inode)
 		goto out;
 
@@ -473,6 +470,8 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
 	d_add(dentry, inode);
 
 out:
+	if (h)
+		sysctl_head_finish(h);
 	sysctl_head_finish(head);
 	return err;
 }