fscrypt.rst: update definition of struct fscrypt_context_v2

Get the copy of the fscrypt_context_v2 definition in the documentation
in sync with the actual definition, which was changed recently by
commit 5b11888 ("fscrypt: support crypto data unit size less than
filesystem block size").

Link: https://lore.kernel.org/r/20231206001901.14371-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

Author: ebiggers

Documentation/filesystems/fscrypt.rst

@@ -1382,7 +1382,8 @@ directory.)  These structs are defined as follows::
             u8 contents_encryption_mode;
             u8 filenames_encryption_mode;
             u8 flags;
-            u8 __reserved[4];
+            u8 log2_data_unit_size;
+            u8 __reserved[3];
             u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
             u8 nonce[FSCRYPT_FILE_NONCE_SIZE];
     };