dm space map metadata: constify dm_space_map structures

Declare dm_space_map structures as const as they are only passed as an
argument to the function memcpy. This argument is of type const void *,
so dm_space_map structures having this property can be declared as
const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   4889	    240	      0	   5129	   1409 dm-space-map-metadata.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   5139	      0	      0	   5139	   1413 dm-space-map-metadata.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>