tcp: Don't store hash bucket in connection structures

Currently when we insert a connection into the hash table, we store its
bucket number so we can find it when removing entries.  However, we can
recompute the hash value from other contents of the structure so we don't
need to store it.  This brings the size of tcp_tap_conn down to 64 bytes
again, which means it will fit in a single cacheline on common machines.

This change also removes a non-obvious constraint that the hash table have
less than twice TCP_MAX_CONNS buckets, because of the way
TCP_HASH_BUCKET_BITS was constructed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tcp_conn.h

@@ -9,8 +9,6 @@
 #ifndef TCP_CONN_H
 #define TCP_CONN_H
 
-#define TCP_HASH_BUCKET_BITS		(TCP_CONN_INDEX_BITS + 1)
-
 /**
  * struct tcp_conn_common - Common fields for spliced and non-spliced
  * @spliced:		Is this a spliced connection?
@@ -32,7 +30,6 @@ extern const char *tcp_common_flag_str[];
  * @events:		Connection events, implying connection states
  * @timer:		timerfd descriptor for timeout events
  * @flags:		Connection flags representing internal attributes
- * @hash_bucket:	Bucket index in connection lookup hash table
  * @retrans:		Number of retransmissions occurred due to ACK_TIMEOUT
  * @ws_from_tap:	Window scaling factor advertised from tap/guest
  * @ws_to_tap:		Window scaling factor advertised to tap/guest
@@ -97,8 +94,6 @@ struct tcp_tap_conn {
 #define ACK_FROM_TAP_DUE	BIT(5)
 
 
-	unsigned int	hash_bucket	:TCP_HASH_BUCKET_BITS;
-
 #define TCP_MSS_BITS			14
 	unsigned int	tap_mss		:TCP_MSS_BITS;
 #define MSS_SET(conn, mss)	(conn->tap_mss = (mss >> (16 - TCP_MSS_BITS)))