tcp: remove one indentation level in tcp_rcv_state_process()

Remove one level of indentation 'introduced' in commit
c3ae62af8e75 (tcp: should drop incoming frames without ACK flag set)

if (true) {
        ...
}

@acceptable variable is a boolean.

This patch is a pure cleanup.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>