dm bufio: get rid of slab cache name allocations

dm-bufio keeps the dm_bufio_cache_names array that holds names of the
slab caches.

Since the commit db265eca ("mm/sl[aou]b: Move duping of slab name to
slab_common.c"), the kernel automatically duplicates the slab cache name
when creating the slab cache, so we no longer have to keep the name
allocated.

Remove the code that allocates the slab names and keeps them around.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>