af_unix: speedup /proc/net/unix
/proc/net/unix has quadratic behavior, and can hold unix_table_lock for
a while if high number of unix sockets are alive. (90 ms for 200k
sockets...)

We already have a hash table, so its quite easy to use it.

Problem is unbound sockets are still hashed in a single hash slot
(unix_socket_table[UNIX_HASH_TABLE])

This patch also spreads unbound sockets to 256 hash slots, to speedup
both /proc/net/unix and unix_diag.

Time to read /proc/net/unix with 200k unix sockets :
(time dd if=/proc/net/unix of=/dev/null bs=4k)

before : 520 secs
after : 2 secs

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
sched: LOAD_FREQ (4*HZ+61) avoids loadavg Moire
LOAD_FREQ is (5*HZ+1) to avoid high load average when idle:
http://kerneltrap.org/mailarchive/linux-kernel/2007/10/3/328568

I suggest (4*HZ+61) for a better distribution.

With some seconds based load (like SSL heartbeats)
and LOAD_FREQ at (5*HZ+1) I see Moire patterns like inverse sawtooth,
since 2 or 3 probes hit the jobs (load increases quickly),
followed by several probes missing it.

A 4.61 sec interval gives optimal distribution over when within a
second a probe is taken, as .61 is close to golden ratio phi 1.618...
(test in http://ripke.com/goldenratio.c).

12*4.61 = 55.32 secs is still close to a minute,
and 13*4.61=59.93 is even closer than the current 12*5.01=60.12
(with exponents EXP_x adjusted to a ratio of 13 instead of 12).

Thanks to Klaus Ripke http://ripke.com/loadavg/moire
https://lkml.org/lkml/2011/11/12/39