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>