af_unix: Optimise hash table layout.

Commit 6dd4142f ("Merge branch 'af_unix-per-netns-socket-hash'") and
commit 51bae889 ("af_unix: Put pathname sockets in the global hash
table.") changed a hash table layout.

  Before:
    unix_socket_table [0   - 255] : abstract & pathname sockets
                      [256 - 511] : unnamed sockets

  After:
    per-netns table   [0   - 255] : abstract & pathname sockets
                      [256 - 511] : unnamed sockets
    bsd_socket_table  [0   - 255] : pathname sockets (sk_bind_node)

Now, while looking up sockets, we traverse the global table for the
pathname sockets and the first half of each per-netns hash table for
abstract sockets, where pathname sockets are also linked.  Thus, the
more pathname sockets we have, the longer we take to look up abstract
sockets.  This characteristic has been there before the layout change,
but we can improve it now.

This patch changes the per-netns hash table's layout so that sockets not
requiring lookup reside in the first half and do not impact the lookup of
abstract sockets.

    per-netns table   [0   - 255] : pathname & unnamed sockets
                      [256 - 511] : abstract sockets
    bsd_socket_table  [0   - 255] : pathname sockets (sk_bind_node)

We have run a test that bind()s 100,000 abstract/pathname sockets for
each, bind()s an abstract socket 100,000 times and measures the time
on __unix_find_socket_byname().  The result shows that the patch makes
each lookup faster.

  Without this patch:
    $ sudo ./funclatency -p 2278 --microseconds __unix_find_socket_byname.isra.44
     usec                : count    distribution
         0 -> 1          : 0        |                                        |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 126      |                                        |
        16 -> 31         : 1438     |*                                       |
        32 -> 63         : 4150     |***                                     |
        64 -> 127        : 9049     |*******                                 |
       128 -> 255        : 37704    |*******************************         |
       256 -> 511        : 47533    |****************************************|

  With this patch:
    $ sudo ./funclatency -p 3648 --microseconds __unix_find_socket_byname.isra.46
     usec                : count    distribution
         0 -> 1          : 109      |                                        |
         2 -> 3          : 318      |                                        |
         4 -> 7          : 725      |                                        |
         8 -> 15         : 2501     |*                                       |
        16 -> 31         : 3061     |**                                      |
        32 -> 63         : 4028     |***                                     |
        64 -> 127        : 9312     |*******                                 |
       128 -> 255        : 51372    |****************************************|
       256 -> 511        : 28574    |**********************                  |

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20220705233715.759-1-kuniyu@amazon.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>