btrfs: rename btrfs_compress_op to btrfs_compress_levels

Since all workspace managers are per-fs, there is no need nor no way to
store them inside btrfs_compress_op::wsm anymore.

With that said, we can do the following modifications:

- Remove zstd_workspace_mananger::ops
  Zstd always grab the global btrfs_compress_op[].
- Remove btrfs_compress_op::wsm member
- Rename btrfs_compress_op to btrfs_compress_levels

This should make it more clear that btrfs_compress_levels structures are
only to indicate the levels of each compress algorithm.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>