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>

security/selinux/hooks.c

@@ -1754,7 +1754,8 @@ static inline void flush_unauthorized_files(struct files_struct * files)
 						get_file(devnull);
 					} else {
 						devnull = dentry_open(dget(selinux_null), mntget(selinuxfs_mount), O_RDWR);
-						if (!devnull) {
+						if (IS_ERR(devnull)) {
+							devnull = NULL;
 							put_unused_fd(fd);
 							fput(file);
 							continue;