dm: Constify struct dm_block_validator

'struct dm_block_validator' are not modified in these drivers.

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
  32047	    920	     16	  32983	   80d7	drivers/md/dm-cache-metadata.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  32075	    896	     16	  32987	   80db	drivers/md/dm-cache-metadata.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>