btrfs: change btrfs_csum_final result param type to u8

csum member of struct btrfs_super_block has array type of u8. It makes
sense that function btrfs_csum_final should be also declared to accept
u8 *. I changed the declaration of method void btrfs_csum_final(u32 crc,
char *result); to void btrfs_csum_final(u32 crc, u8 *result);

Signed-off-by: Domagoj Tršan <domagoj.trsan@gmail.com>
[ changed cast to u8 at several call sites ]
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 66c1d65..e6300d0 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3102,7 +3102,7 @@
 
 	kaddr = kmap_atomic(page);
 	csum = btrfs_csum_data(kaddr + pgoff, csum,  len);
-	btrfs_csum_final(csum, (char *)&csum);
+	btrfs_csum_final(csum, (u8 *)&csum);
 	if (csum != csum_expected)
 		goto zeroit;