procfs: add num_to_str() to speed up /proc/stat

== stat_check.py
num = 0
with open("/proc/stat") as f:
        while num < 1000 :
                data = f.read()
                f.seek(0, 0)
                num = num + 1
==

perf shows

    20.39%  stat_check.py  [kernel.kallsyms]    [k] format_decode
    13.41%  stat_check.py  [kernel.kallsyms]    [k] number
    12.61%  stat_check.py  [kernel.kallsyms]    [k] vsnprintf
    10.85%  stat_check.py  [kernel.kallsyms]    [k] memcpy
     4.85%  stat_check.py  [kernel.kallsyms]    [k] radix_tree_lookup
     4.43%  stat_check.py  [kernel.kallsyms]    [k] seq_printf

This patch removes most of calls to vsnprintf() by adding num_to_str()
and seq_print_decimal_ull(), which prints decimal numbers without rich
functions provided by printf().

On my 8cpu box.
== Before patch ==
[root@bluextal test]# time ./stat_check.py

real    0m0.150s
user    0m0.026s
sys     0m0.121s

== After patch ==
[root@bluextal test]# time ./stat_check.py

real    0m0.055s
user    0m0.022s
sys     0m0.030s

[akpm@linux-foundation.org: remove incorrect comment, use less statck in num_to_str(), move comment from .h to .c, simplify seq_put_decimal_ull()]
[andrea@betterlinux.com: avoid breaking the ABI in /proc/stat]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrea Righi <andrea@betterlinux.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Turner <pjt@google.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
backported from Linux 3.4-rc4 to Linux 3.0.x by faux123
vsprintf: optimize decimal conversion (again)

    This patch is based upon an original from Michal Nazarewicz.

    Previous code was using optimizations which were developed
    to work well even on narrow-word CPUs (by today's standards).
    But Linux runs only on 32-bit and wider CPUs. We can use that.

    First: using 32x32->64 multiply and trivial 32-bit shift,
    we can correctly divide by 10 much larger numbers, and thus
    we can print groups of 9 digits instead of groups of 5 digits.

    Next: there are two algorithms to print larger numbers.
    One is generic: divide by 1000000000 and repeatedly print
    groups of (up to) 9 digits. It's conceptually simple,
    but requires an (unsigned long long) / 1000000000 division.
    Second algorithm splits 64-bit unsigned long long into 16-bit chunks,
    manipulates them cleverly and generates groups of 4 decimal digits.
    It so happens that it does NOT require long long division.

    If long is > 32 bits, division of 64-bit values is relatively easy,
    and we will use the first algorithm.
    If long long is > 64 bits (strange architecture with VERY large long long),
    second algorithm can't be used, and we again use the first one.
    Else (if long is 32 bits and long long is 64 bits) we use second one.

    And third: there is a simple optimization which takes fast path
    not only for zero as was done before, but for all one-digit numbers.

    In all tested cases new code is faster than old one, in many cases by 30%,
    in few cases by more than 50% (for example, on x86-32, conversion of 12345678).
    Code growth is ~0 in 32-bit case and ~130 bytes in 64-bit case.

    Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
    Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
    Cc: Douglas W Jones <jones@cs.uiowa.edu>a
    backported from Linux 3.4+ to Linux 3.0 by faux123