fscrypt: document struct fscrypt_operations

Document all fields of struct fscrypt_operations so that it's more clear
what filesystems that use (or plan to use) fs/crypto/ need to implement.

Link: https://lore.kernel.org/r/20210729043728.18480-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>