btrfs: Use scrub_checksum_data and scrub_checksum_tree_block for scrub_recheck_block_checksum

We can use existing scrub_checksum_data() and scrub_checksum_tree_block()
for scrub_recheck_block_checksum(), instead of write duplicated code.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>