UPSTREAM: fscrypt: add FS_IOC_GET_ENCRYPTION_NONCE ioctl

Add an ioctl FS_IOC_GET_ENCRYPTION_NONCE which retrieves the nonce from
an encrypted file or directory.  The nonce is the 16-byte random value
stored in the inode's encryption xattr.  It is normally used together
with the master key to derive the inode's actual encryption key.

The nonces are needed by automated tests that verify the correctness of
the ciphertext on-disk.  Except for the IV_INO_LBLK_64 case, there's no
way to replicate a file's ciphertext without knowing that file's nonce.

The nonces aren't secret, and the existing ciphertext verification tests
in xfstests retrieve them from disk using debugfs or dump.f2fs.  But in
environments that lack these debugging tools, getting the nonces by
manually parsing the filesystem structure would be very hard.

To make this important type of testing much easier, let's just add an
ioctl that retrieves the nonce.

Link: https://lore.kernel.org/r/20200314205052.93294-2-ebiggers@kernel.org


Reviewed-by: Theodore Ts'o <tytso@mit.edu>
(cherry picked from commit e98ad464750c0894bc560d10503dae8ff90ccdac)
Bug: 151100202
Change-Id: Ieef2c57b9257ae38eb2e5c1a018ca6f325bb62dd
Signed-off-by: Eric Biggers <ebiggers@google.com>