ext4 crypto: fix to check feature status before get policy

When getting fscrypt policy via EXT4_IOC_GET_ENCRYPTION_POLICY, if
encryption feature is off, it's better to return EOPNOTSUPP instead of
ENODATA, so let's add ext4_has_feature_encrypt() to do the check for
that.

This makes it so that all fscrypt ioctls consistently check for the
encryption feature, and makes ext4 consistent with f2fs in this regard.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
[EB - removed unneeded braces, updated the documentation, and
      added more explanation to commit message]
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/filesystems/fscrypt.rst

@@ -562,7 +562,8 @@ FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:
   or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
   (try FS_IOC_GET_ENCRYPTION_POLICY instead)
 - ``EOPNOTSUPP``: the kernel was not configured with encryption
-  support for this filesystem
+  support for this filesystem, or the filesystem superblock has not
+  had encryption enabled on it
 - ``EOVERFLOW``: the file is encrypted and uses a recognized
   encryption policy version, but the policy struct does not fit into
   the provided buffer

fs/ext4/ioctl.c

@@ -1113,6 +1113,8 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 #endif
 	}
 	case EXT4_IOC_GET_ENCRYPTION_POLICY:
+		if (!ext4_has_feature_encrypt(sb))
+			return -EOPNOTSUPP;
 		return fscrypt_ioctl_get_policy(filp, (void __user *)arg);
 
 	case FS_IOC_GET_ENCRYPTION_POLICY_EX: