staging: lustre: tidy up ldlm_resource_putref()

1/ the return value of ldlm_resource_putref() is never
  used, so change it to return 'void'.
2/ Move all of the code to run on the last putref to
   __ldlm_resource_putref_final().  This means a lock
   is taken in one function and dropped in another, but
   that isn't too uncommon, and will disappear in a future
   patch.
   Now that the code it together, it becomes apparent that
   we are dropping a ref on the namespace *before* the last
   use.  So keep the ref until after.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>