btrfs: sysfs: don't leak memory when failing add fsid

A failed call to kobject_init_and_add() must be followed by a call to
kobject_put().  Currently in the error path when adding fs_devices we
are missing this call.  This could be fixed by calling
btrfs_sysfs_remove_fsid() if btrfs_sysfs_add_fsid() returns an error or
by adding a call to kobject_put() directly in btrfs_sysfs_add_fsid().
Here we choose the second option because it prevents the slightly
unusual error path handling requirements of kobject from leaking out
into btrfs functions.

Add a call to kobject_put() in the error path of kobject_add_and_init().
This causes the release method to be called if kobject_init_and_add()
fails.  open_tree() is the function that calls btrfs_sysfs_add_fsid()
and the error code in this function is already written with the
assumption that the release method is called during the error path of
open_tree() (as seen by the call to btrfs_sysfs_remove_fsid() under the
fail_fsdev_sysfs label).

Cc: stable@vger.kernel.org # v4.4+
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -825,7 +825,12 @@ int btrfs_sysfs_add_fsid(struct btrfs_fs_devices *fs_devs,
 	fs_devs->fsid_kobj.kset = btrfs_kset;
 	error = kobject_init_and_add(&fs_devs->fsid_kobj,
 				&btrfs_ktype, parent, "%pU", fs_devs->fsid);
+	if (error) {
+		kobject_put(&fs_devs->fsid_kobj);
 		return error;
+	}
+
+	return 0;
 }
 
 int btrfs_sysfs_add_mounted(struct btrfs_fs_info *fs_info)