net: sk_msg: Simplify sk_psock initialization

Initializing psock->sk_proto and other saved callbacks is only
done in sk_psock_update_proto, after sk_psock_init has returned.
The logic for this is difficult to follow, and needlessly complex.

Instead, initialize psock->sk_proto whenever we allocate a new
psock. Additionally, assert the following invariants:

* The SK has no ULP: ULP does it's own finagling of sk->sk_prot
* sk_user_data is unused: we need it to store sk_psock

Protect our access to sk_user_data with sk_callback_lock, which
is what other users like reuseport arrays, etc. do.

The result is that an sk_psock is always fully initialized, and
that psock->sk_proto is always the "original" struct proto.
The latter allows us to use psock->sk_proto when initializing
IPv6 TCP / UDP callbacks for sockmap.

Signed-off-by: Lorenz Bauer <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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