btrfs: scrub: switch scrub_simple_mirror() to scrub_stripe infrastruc…

…ture

Switch scrub_simple_mirror() to the new scrub_stripe infrastructure.

Since scrub_simple_mirror() is the core part of scrub (only RAID56
P/Q stripes don't utilize it), we can get rid of a big chunk of code,
mostly scrub_extent(), scrub_sectors() and directly called functions.

There is a functionality change:

- Scrub speed throttle now only affects read on the scrubbing device
  Writes (for repair and replace), and reads from other mirrors won't
  be limited by the set limits.

Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>