staging: lustre: use strim instead of cfs_trimwhite.

Linux lib provides identical functionality to cfs_trimwhite,
so discard that code and use the standard.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>