poll: rust: allow poll_table ptrs to be null

It's possible for a poll_table to be null. This can happen if an
end-user just wants to know if a resource has events right now without
registering a waiter for when events become available. Furthermore,
these null pointers should be handled transparently by the API, so we
should not change `from_ptr` to return an `Option`. Thus, change
`PollTable` to wrap a raw pointer rather than use a reference so that
you can pass null.

Comments mentioning `struct poll_table` are changed to just `poll_table`
since `poll_table` is a typedef. (It's a typedef because it's supposed
to be opaque.)

Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>