staging: lustre: discard cfs_duration_sec()

cfs_duration_sec() simply divides by HZ.
It is mostly used to report durations in debug messages.
Remove and just use X/HZ.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>