staging: lustre: fix return type of lo_release.

The return type of block_device_operations.release() changed to void in commit
db2a144b.

Found with the following Coccinelle patch:
<smpl>
@has_release_func@
identifier i;
identifier release_func;
@@
struct block_device_operations i = {
 .release = release_func
};

@depends on has_release_func@
identifier has_release_func.release_func;
@@
- int
+ void
release_func(...) {
...
- return ...;
}
</smpl>

Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>