Documentations: Analyze heavily used Networking related structs

Analyzed a few structs in the networking stack by looking at variables
within them that are used in the TCP/IP fast path.

Fast path is defined as TCP path where data is transferred from sender to
receiver unidirectionally. It doesn't include phases other than
TCP_ESTABLISHED, nor does it look at error paths.

We hope to re-organizing variables that span many cachelines whose fast
path variables are also spread out, and this document can help future
developers keep networking fast path cachelines small.

Optimized_cacheline field is computed as
(Fastpath_Bytes/L3_cacheline_size_x86), and not the actual organized
results (see patches to come for these).

Investigation is done on 6.5

Name	                Struct_Cachelines  Cur_fastpath_cache Fastpath_Bytes Optimized_cacheline
tcp_sock	        42 (2664 Bytes)	   12   		396		8
net_device	        39 (2240 bytes)	   12			234		4
inet_sock	        15 (960 bytes)	   14			922		14
Inet_connection_sock	22 (1368 bytes)	   18			1166		18
Netns_ipv4 (sysctls)	12 (768 bytes)     4			77		2
linux_mib	        16 (1060)	   6			104		2

Note how there isn't much improvement space for inet_sock and
Inet_connection_sock because sk and icsk_inet respectively takes up so
much of the struct that rest of the variables become a small portion of
the struct size.

So, we decided to reorganize tcp_sock, net_device, netns_ipv4

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Coco Li <lixiaoyan@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>