staging: lustre: cfs_time_sub() must return unsigned long

It looks like someone ment for it to return this value (there's a cast),
but the funtion says 'long' which causes problems when comparing values
from this function.

Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: hpdd-discuss <hpdd-discuss@lists.01.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>