iommufd: Constify struct dma_buf_attach_ops

iommufd: Constify struct dma_buf_attach_ops

'struct dma_buf_attach_ops' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  81096	  13899	    192	  95187	  173d3	drivers/iommu/iommufd/pages.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  81160	  13835	    192	  95187	  173d3	drivers/iommu/iommufd/pages.o

Link: https://patch.msgid.link/r/67e9126bbffa1d5c05124773a8dd4a3493be77ac.1772139886.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>