He hasns't reguarly been first choice, and signings like Bailly and Lindelof were supposed to relegate him. I'm sure he's unhappy at being dropped to fourth, but he's fought back before and did well. I doubt he decided to leave three matches into the season rather than try and fight for his place at least until January.You doubt that Smalling, who has been first choice for 6 years, is a bit peeved at clearly being relegated to fourth choice at best?
I'm fairly certain Smalling is not at all happy about the situation and it's not about "giving up", if you see yourself as a first team starter and you've been made fourth choice, you look for a move if you're ambitious.
Smalling is doing the right thing here in my opinion if he does complete this move.