tcp: Increase TCBHASHSIZE from 512 to 4096

In netperf TCP_CC test, in_pcblookup_hash_locked() is observed as high as
~50% cpu usage. Increasing TCBHASHSIZE to 4096 imporves TCP_CC by 57%
while drops cpu usage to ~13%.

Before:
TCBHASHSIZE 512  -> 57% cpu usage -> 7K  Rate/Sec

After:
TCBHASHSIZE 1024 -> 40% cpu usage -> 9K  Rate/sec
TCBHASHSIZE 4096 -> 13% cpu usage -> 11K Rate/sec

Signed-off-by: Asias He <asias@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>