staging/lustre: use 'long' return type for cfs_duration_sec()
The cfs_duration_sec() converts a relative jiffies value into seconds,
and returns that number as a time_t. We know that a 32-bit type is
enough here, because the result is order of magnitudes smaller than
the difference in jiffies that is also expressed as a 'long', so
we can safely replace the time_t type with long as well.