ktime: introduce ktime_to_ms()

To report ktime statistics to user space in milliseconds, a new helper
is required.

When considering how to do this conversion, I didn't immediately see
why the extra step of converting ktime to a timeval was needed.  To
make that more clear, introduce a couple of large comments.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>