btrfs: scrub: remove scrub_block and scrub_sector structures

btrfs: scrub: remove scrub_block and scrub_sector structures

Those two structures are used to represent a bunch of sectors for scrub,
but now they are fully replaced by scrub_stripe in one go, so we can
remove them. This involves:

- structure scrub_block
- structure scrub_sector

- structure scrub_page_private
- function attach_scrub_page_private()
- function detach_scrub_page_private()
  Now we no longer need to use page::private to handle subpage.

- function alloc_scrub_block()
- function alloc_scrub_sector()
- function scrub_sector_get_page()
- function scrub_sector_get_page_offset()
- function scrub_sector_get_kaddr()
- function bio_add_scrub_sector()

- function scrub_checksum_data()
- function scrub_checksum_tree_block()
- function scrub_checksum_super()
- function scrub_check_fsid()
- function scrub_block_get()
- function scrub_block_put()
- function scrub_sector_get()
- function scrub_sector_put()
- function scrub_bio_end_io()
- function scrub_block_complete()
- function scrub_add_sector_to_rd_bio()

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