RDMA/hfi1: Constify struct mmu_rb_ops

RDMA/hfi1: Constify struct mmu_rb_ops

'struct mmu_rb_ops' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  10879	    164	      0	  11043	   2b23	drivers/infiniband/hw/hfi1/pin_system.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  10907	    140	      0	  11047	   2b27	drivers/infiniband/hw/hfi1/pin_system.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/b826dd05eefa5f4d6a7a1b4d191eaf37c714ed04.1719259997.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>