security: testing the wrong variable in create_by_name()

There is a typo here.  We should be testing "*dentry" instead of
"dentry".  If "*dentry" is an ERR_PTR, it gets dereferenced in either
mkdir() or create() which would cause an OOPs.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: James Morris <jmorris@namei.org>