ubifs: wire up FS_IOC_GET_ENCRYPTION_NONCE

This new ioctl retrieves a file's encryption nonce, which is useful for
testing.  See the corresponding fs/crypto/ patch for more details.

Link: https://lore.kernel.org/r/20200314205052.93294-5-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>