rust: types: add Opaque::try_ffi_init

This will be used by the miscdevice abstractions, as the C function
`misc_register` is fallible.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20241001-b4-miscdevice-v2-1-330d760041fa@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>