bcachefs: __bio_compress() fix up.

A single block can't be compressed, so it's incompressible.
This stops rebalance repeatably marking extents as uncompressed.
Signed-off-by: Daniel Hill <daniel@gluo.nz>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/compress.c

@@ -377,7 +377,7 @@ static unsigned __bio_compress(struct bch_fs *c,
 
 	/* If it's only one block, don't bother trying to compress: */
 	if (src->bi_iter.bi_size <= c->opts.block_size)
-		return 0;
+		return BCH_COMPRESSION_TYPE_incompressible;
 
 	dst_data = bio_map_or_bounce(c, dst, WRITE);
 	src_data = bio_map_or_bounce(c, src, READ);