Merge branch 'bpf-skmsg-simplify-restore'

Jakub Sitnicki says:

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This series has been split out from "Extend SOCKMAP to store listening
sockets" [0]. I think it stands on its own, and makes the latter series
smaller, which will make the review easier, hopefully.

The essence is that we don't need to do a complicated dance in
sk_psock_restore_proto, if we agree that the contract with tcp_update_ulp
is to restore callbacks even when the socket doesn't use ULP. This is what
tcp_update_ulp currently does, and we just make use of it.

Series is accompanied by a test for a particularly tricky case of restoring
callbacks when we have both sockmap and tls callbacks configured in
sk->sk_prot.

[0] https://lore.kernel.org/bpf/20200127131057.150941-1-jakub@cloudflare.com/
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>