btrfs: raid56: prepare recover_vertical() to support bs > ps cases

btrfs: raid56: prepare recover_vertical() to support bs > ps cases

Currently recover_vertical() assumes that every fs block can be mapped
by one page, this is blocking bs > ps support for raid56.

Prepare recover_vertical() to support bs > ps cases by:

- Introduce recover_vertical_step() helper
  Which will recover a full step (min(PAGE_SIZE, sectorsize)).

  Now recover_vertical() will do the error check for the specified
  sector, do the recover step by step, then do the sector verification.

- Fix a spelling error of get_rbio_vertical_errors()
  The old name has a typo: "veritical".

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