We should reset sblock->xxx_error stats before calling
scrub_recheck_block_checksum().

Current code run correctly because all sblock are allocated by
k[cz]alloc(), and the error stats are not got changed.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>