staging/lustre/llite: constify export_operations structures

This export_operations structure is never modified, so declare it as const.
Most other structures of this type are already const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>