BACKPORT: fscrypt: rename FS_KEY_DERIVATION_NONCE_SIZE

The name "FS_KEY_DERIVATION_NONCE_SIZE" is a bit outdated since due to
the addition of FSCRYPT_POLICY_FLAG_DIRECT_KEY, the file nonce may now
be used as a tweak instead of for key derivation.  Also, we're now
prefixing the fscrypt constants with "FSCRYPT_" instead of "FS_".

Therefore, rename this constant to FSCRYPT_FILE_NONCE_SIZE.

Bug: 254441685
Link: https://lore.kernel.org/r/20200708215722.147154-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>
[Lee: Back-ported to solve a dependency issue]
(cherry picked from commit 1d6217a4f9905917ee63315c8ea3d63833792f51)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I5eae506505d9432b95dabf620b79a6b8ca1e0d6a