btrfs: turn checksum type define into an enum

Turn the checksum type definition into a enum. This eases later addition
of new checksums.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs_tree.h

@@ -300,7 +300,9 @@
 #define BTRFS_CSUM_SIZE 32
 
 /* csum types */
-#define BTRFS_CSUM_TYPE_CRC32	0
+enum btrfs_csum_type {
+	BTRFS_CSUM_TYPE_CRC32	= 0,
+};
 
 /*
  * flags definitions for directory entry item type