ext4: 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/20200314205052.93294-3-ebiggers@kernel.org


Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Bug: 151100202
Change-Id: I85350aed66285b92444d37c8cd840fb03d2ca25d
Signed-off-by: Eric Biggers <ebiggers@google.com>
Git-commit: 7ec9f3b47aba0fe715bf3472ed80e91c37970363
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[neersoni@codeaurora.org: Back port the change]
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>