fs: btrfs: Use LZO_LEN to replace immediate number

Just a cleanup. These immediate numbers make my eyes hurt.
Signed-off-by: NQu Wenruo <wqu@suse.com>
Cc: Marek Behun <marek.behun@nic.cz>
Reviewed-by: NMarek Behún <marek.behun@nic.cz>

fs/btrfs/compression.c

@@ -12,36 +12,38 @@
 #include <u-boot/zlib.h>
 #include <asm/unaligned.h>
 
+/* Header for each segment, LE32, recording the compressed size */
+#define LZO_LEN		4
 static u32 decompress_lzo(const u8 *cbuf, u32 clen, u8 *dbuf, u32 dlen)
 {
 	u32 tot_len, in_len, res;
 	size_t out_len;
 	int ret;
 
-	if (clen < 4)
+	if (clen < LZO_LEN)
 		return -1;
 
 	tot_len = le32_to_cpu(get_unaligned((u32 *)cbuf));
-	cbuf += 4;
-	clen -= 4;
-	tot_len -= 4;
+	cbuf += LZO_LEN;
+	clen -= LZO_LEN;
+	tot_len -= LZO_LEN;
 
 	if (tot_len == 0 && dlen)
 		return -1;
-	if (tot_len < 4)
+	if (tot_len < LZO_LEN)
 		return -1;
 
 	res = 0;
 
-	while (tot_len > 4) {
+	while (tot_len > LZO_LEN) {
 		in_len = le32_to_cpu(get_unaligned((u32 *)cbuf));
-		cbuf += 4;
-		clen -= 4;
+		cbuf += LZO_LEN;
+		clen -= LZO_LEN;
 
-		if (in_len > clen || tot_len < 4 + in_len)
+		if (in_len > clen || tot_len < LZO_LEN + in_len)
 			return -1;
 
-		tot_len -= 4 + in_len;
+		tot_len -= (LZO_LEN + in_len);
 
 		out_len = dlen;
 		ret = lzo1x_decompress_safe(cbuf, in_len, dbuf, &out_len);