ubifs: Use fscrypt ioctl() helpers

Commit db717d8e

 ("fscrypto: move ioctl processing more fully into
common code") moved ioctl() related functions into fscrypt and offers
us now a set of helper functions.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: David Gstir <david@sigma-star.at>