dm-cache: fix warnings about duplicate slab caches

The commit 4c395296 adds a warning about duplicate cache names if
CONFIG_DEBUG_VM is selected. These warnings are triggered by the dm-cache
code.

The dm-cache code allocates a slab cache for each device. This commit
changes it to allocate just one slab cache in the module init function.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 4c395296 ("slab: Warn on duplicate cache names when DEBUG_VM=y")