selinux: fix dentry_open() error check

The return value of dentry_open() shoud be checked by IS_ERR().

Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: James Morris <jmorris@namei.org>