UPSTREAM: 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


Change-Id: I8f15e873b41e036bc039119a579beac1b78f937b
Bug: 151100202
(cherry picked from commit 861261f2a9cc488c845fc214d9035f7a11094591)
Signed-off-by: Eric Biggers <ebiggers@google.com>