This patch introduces performance histogram record keeping for block 
I/O, used for performance tuning.  It is turned off by default.

When turned on, you simply do something like:

# cat /sys/block/sda/read_request_histo
rows = bytes columns = ms
         10      20      50      100     200     500     1000    2000
    2048 5       0       0       0       0       0       0       0
    4096 0       0       0       0       0       0       0       0
    8192 17231   135     41      10      0       0       0       0
   16384 4400    24      6       2       0       0       0       0
   32768 2897    34      4       4       0       0       0       0
   65536 7089    87      5       1       2       0       0       0
#

Signed-off-by:	Edward A. Falk <efalk@google.com>