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 5b118884 ("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>