btrfs: zstd: remove indirect calls for local functions

While calling functions inside zstd, we don't need to use the
indirection provided by the workspace_manager. Forward declarations are
added to maintain the function order of btrfs_compress_op.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>