dccp/tcp: Remove NULL check for prev_saddr in inet_bhash2_update_saddr().

When we call inet_bhash2_update_saddr(), prev_saddr is always non-NULL.
Let's remove the unnecessary test.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>