datapath: Fix flow time used computation.

datapath: Fix flow time used computation.

The current reporting of flow last used time has two issues that
cause it to incorrectly report the system monotonic time when the
flow was last used.

The first is that it simply converts the stored jiffies value to
milliseconds by scaling with a constant.  This does not work because
jiffies is not zero based and can wrap around on 32-bit platforms.

The second is there is no guarantee that jiffies advances at the
same rate as the RTC based monotonic time that userspace uses.
A variety of factors can cause differences, including system suspend
and clock drift.  These are not too important for relatively short
time periods such as the duration of the flow (nor is the flow timing
precision of extreme importance).  However, when the time being
measured is the duration since system boot (assuming that the above
issues had been addressed) the difference can become significant.

This addresses both issues by restoring behavior similar to the
previous method of computing the flow used time, though in a
slightly different form to reflect the needs of the Netlink code.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>