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.

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>