Btrfs: switch to crc32c instead of sha256

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 92a6078..d8caeba 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -30,6 +30,8 @@
 
 /* 32 bytes in various csum fields */
 #define BTRFS_CSUM_SIZE 32
+/* four bytes for CRC32 */
+#define BTRFS_CRC32_SIZE 4
 
 /*
  * the key defines the order in the tree, and so it also defines (optimal)
@@ -235,7 +237,7 @@
 } __attribute__ ((__packed__));
 
 struct btrfs_csum_item {
-	u8 csum[BTRFS_CSUM_SIZE];
+	u8 csum;
 } __attribute__ ((__packed__));
 
 struct btrfs_device_item {