staging/lustre: use jiffies_to_*() instead of cfs_duration_usec

The cfs_duration_usec() function has a timeval as its output, which we
want to avoid in general because of the y2038 problem.

There are only two locations remaining in lustre, so we can for now
replace one with jiffies_to_timeval(), which is a generic kernel function
that does the same thing, the other can just use jiffies_to_usecs()
and completely avoid the timeval.

This is not a full solution yet, but it's a small step that lets us
build a larger portion of lustre without this reference to timeval in
a header file, and avoid triggering automated checking tools that wants
to warn about timeval.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>