lustre: constify inode_operations structures

The inode_operations structures are never modified, so declare them
as const, like all the other inode_operations structures.

Done with the help of Coccinelle.

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