btrfs: scrub: silence an uninitialized variable warning

It's basically harmless if "ref_level" isn't initialized since it's only
used for an error message, but it causes a static checker warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -611,7 +611,7 @@ static void scrub_print_warning(const char *errstr, struct scrub_block *sblock)
 	u64 flags = 0;
 	u64 ref_root;
 	u32 item_size;
-	u8 ref_level;
+	u8 ref_level = 0;
 	int ret;
 
 	WARN_ON(sblock->page_count < 1);