btrfs: compression: pass type to btrfs_get_workspace

We can infer the workspace_manager from type and the type will be used
in the following patch to call a common helper for alloc_workspace.

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