BUG/MINOR: freq-ctr: make swrate_add() support larger values

BUG/MINOR: freq-ctr: make swrate_add() support larger values

Reinhard Vicinus reported that the reported average response times cannot
be larger than 16s due to the double multiply being performed by
swrate_add() which causes an overflow very quickly. Indeed, with N=512,
the highest average value is 16448.

One solution proposed by Reinhard is to turn to long long, but this
involves 64x64 multiplies and 64->32 divides, which are extremely
expensive on 32-bit platforms.

There is in fact another way to avoid the overflow without using larger
integers, it consists in avoiding the multiply using the fact that
x*(n-1)/N = x-(x/N).

Now it becomes possible to store average values as large as 8.4 millions,
which is around 2h18mn.

Interestingly, this improvement also makes the code cheaper to execute
both on 32 and on 64 bit platforms :

Before :

00000000 <swrate_add>:
   0:   8b 54 24 04             mov    0x4(%esp),%edx
   4:   8b 0a                   mov    (%edx),%ecx
   6:   89 c8                   mov    %ecx,%eax
   8:   c1 e0 09                shl    $0x9,%eax
   b:   29 c8                   sub    %ecx,%eax
   d:   8b 4c 24 0c             mov    0xc(%esp),%ecx
  11:   c1 e8 09                shr    $0x9,%eax
  14:   01 c8                   add    %ecx,%eax
  16:   89 02                   mov    %eax,(%edx)

After :

00000020 <swrate_add>:
  20:   8b 4c 24 04             mov    0x4(%esp),%ecx
  24:   8b 44 24 0c             mov    0xc(%esp),%eax
  28:   8b 11                   mov    (%ecx),%edx
  2a:   01 d0                   add    %edx,%eax
  2c:   81 c2 ff 01 00 00       add    $0x1ff,%edx
  32:   c1 ea 09                shr    $0x9,%edx
  35:   29 d0                   sub    %edx,%eax
  37:   89 01                   mov    %eax,(%ecx)

This fix may be backported to 1.6.