dm cache metadata: verify cache has blocks in blocks_are_clean_separate_dirty()

Otherwise dm_bitset_cursor_begin() return -ENODATA.  Other calls to
dm_bitset_cursor_begin() have similar negative checks.

Fixes inability to create a cache in passthrough mode (even though doing
so makes no sense).

Fixes: 0d963b6e ("dm cache metadata: fix metadata2 format's blocks_are_clean_separate_dirty")
Cc: stable@vger.kernel.org
Reported-by: David Teigland <teigland@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-metadata.c

@@ -930,6 +930,10 @@ static int blocks_are_clean_separate_dirty(struct dm_cache_metadata *cmd,
 	bool dirty_flag;
 	*result = true;
 
+	if (from_cblock(cmd->cache_blocks) == 0)
+		/* Nothing to do */
+		return 0;
+
 	r = dm_bitset_cursor_begin(&cmd->dirty_info, cmd->dirty_root,
 				   from_cblock(cmd->cache_blocks), &cmd->dirty_cursor);
 	if (r) {