btrfs: scrub: remove the old writeback infrastructure

Since the whole scrub path has been switched to scrub_stripe based
solution, the old writeback path can be removed completely, which
involves:

- scrub_ctx::wr_curr_bio member
- scrub_ctx::flush_all_writes member
- function scrub_write_block_to_dev_replace()
- function scrub_write_sector_to_dev_replace()
- function scrub_add_sector_to_wr_bio()
- function scrub_wr_submit()
- function scrub_wr_bio_end_io()
- function scrub_wr_bio_end_io_worker()

And one more function needs to be exported temporarily:

- scrub_sector_get()
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.h

@@ -19,5 +19,6 @@ struct scrub_sector;
 int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u8 *csum);
 int scrub_add_sector_to_rd_bio(struct scrub_ctx *sctx,
 			       struct scrub_sector *sector);
+void scrub_sector_get(struct scrub_sector *sector);
 
 #endif