io_latency: add io_latency.h 

include/linux/io_latency.h contains iolatency related structure/function
definitions.

Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
io_latency: add iolatency stats in block/scsi layer

1. Extend struct request and request_queue to contain iolatency related field
2. do iolatency stats in get_request_wait()/blk_finish_request()/scsi_dispatch_cmd()

Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
io_latency: export proc interface

Add proc interface for iolatency under /proc/io-latency/sdX. There're 3
entries controlling the behavior of iolatency:
1. enable_iolatency: disable/enable the RT stats for LLD/device
2. enable_soft_iolatency: disable/enable the RT stats for software
layer(mainly cost in block and scsi layer)
3. enable_use_us: diable/enable millisecond-level stats. Notice that it
can only be modified when enable_iolatency=0 and enable_soft_iolatency=0

Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
Merge pull request #50 from shazj99/iolatency

Add Iolatency

Thanks, Applied.