netfilter: ctnetlink: use expect instead of master tuple in get operation

Use the expect tuple (if possible) instead of the master tuple for
the get operation. If two or more expectations come from the same
master, the returned expectation may not be the one that user-space
is requesting.

This is how it works for the expect deletion operation.

Although I think that nobody has been seriously using this. We
accept both possibilities, using the expect tuple if possible.
I decided to do it like this to avoid breaking backward
compatibility.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>