btrfs: compression: let workspace manager init take only the type

With the access to the workspace structures, we can look it up together
with the compression ops inside the workspace manager init helper.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/zlib.c

@@ -31,7 +31,7 @@ static struct workspace_manager wsm;
 
 static void zlib_init_workspace_manager(void)
 {
-	btrfs_init_workspace_manager(&wsm, &btrfs_zlib_compress);
+	btrfs_init_workspace_manager(BTRFS_COMPRESS_ZLIB);
 }
 
 static void zlib_cleanup_workspace_manager(void)