selinux: fix dentry_open() error check

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

Cc: Stephen Smalley <[e-mail]>
Cc: James Morris <[e-mail]>
Signed-off-by: Akinobu Mita <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

security/selinux/hooks.c

···
 						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;