btrfs-progs: receive: properly report lack of zstd support

If zstd is not compiled in then a stream fails with a generic error
message:

ERROR: unknown compression: 2

Where BTRFS_ENCODED_IO_COMPRESSION_ZSTD is 2 and there's a case for that
but behind the '#if COMPRESSION_ZSTD'.

Signed-off-by: David Sterba <dsterba@suse.com>

cmds/receive.c

@@ -1184,8 +1184,8 @@ static int decompress_and_write(struct btrfs_receive *rctx,
 		if (ret)
 			goto out;
 		break;
-#if COMPRESSION_ZSTD
 	case BTRFS_ENCODED_IO_COMPRESSION_ZSTD:
+#if COMPRESSION_ZSTD
 		ret = decompress_zstd(rctx, encoded_data, encoded_len,
 				      unencoded_data, unencoded_len);
 		if (ret)