ext4: avoid trying to kfree an ERR_PTR pointer

Thanks to Dan Carpenter for extending smatch to find bugs like this.
(This was found using a development version of smatch.)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[lizf: Backported to 3.4:
- s/new.bh/new_bh/
- drop the change to ext4_cross_rename()]
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>