btrfs: scrub: rename scrub_page to scrub_sector

Since the subpage support of scrub, scrub_sector is in fact just
representing one sector.

Thus the name scrub_page is no longer correct, rename it to
scrub_sector.

This also involves the following renames:

- spage -> sector
  Normally we would just replace "page" with "sector" and result
  something like "ssector".
  But the repeating 's' is not really eye friendly.

  So here we just simple use "sector", as there is nothing from MM layer
  called "sector" to cause any confusion.

- scrub_parity::spages -> sectors_list
  Normally we use plural to indicate an array, not a list.
  Rename it to @sectors_list to be more explicit on the list part.

- Also reformat and update comments that get changed
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>