supplement inflight I/O requests

[Hunter1016]

Inflight I/O requests count I/O requests issued to the device driver but have not yet completed, which are useful to analyze system I/O status. More than this, since plenty of processes remain in D status for a long time come from I/O troubles, inflight I/O values also help to examine these processes.

Signed-off-by: Teng Hu [email protected]