fscrypt-crypt-util: fix maximum IV size

fscrypt-crypt-util: fix maximum IV size

In commit 65cd8e8a8e81 ("fscrypt-crypt-util: fix IV incrementing for
--iv-ino-lblk-32") I mistakenly decreased the size of fscrypt_iv to 24
bytes, which is the most that is explicitly needed by any of the IV
generation methods.  However, Adiantum encryption takes a 32-byte IV, so
the buffer still needs to be 32 bytes, with any extra bytes zeroed.

So restore the size to 32 bytes.

This fixes a buffer overread that caused generic/550 and generic/584 to
sometimes fail, depending on the build of the fscrypt-crypt-util binary.
(Most of the time it still worked by chance.)

Fixes: 65cd8e8a8e81 ("fscrypt-crypt-util: fix IV incrementing for --iv-ino-lblk-32")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>