Staging: lustre: obdclass: lu_object: Declare local functions as static

Declare lu_dev_add_linkage and lu_object_find as static since they are
used only in this particular file. Also remove the corresponding
declarations from header file.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>