f2fs: 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/20200314205052.93294-4-ebiggers@kernel.org


Bug: 151100202
Change-Id: I26037efc77774c04951da887d8557a15ce6a23ed
Signed-off-by: Eric Biggers <ebiggers@google.com>
Git-commit: ee446e1af4ff90945e2273ae0af322b8b745b8cc
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>