btrfs: scrub: switch scrub_simple_mirror() to scrub_stripe infrastructure

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

fs/btrfs/scrub.h

@@ -13,14 +13,4 @@ int btrfs_scrub_cancel_dev(struct btrfs_device *dev);
 int btrfs_scrub_progress(struct btrfs_fs_info *fs_info, u64 devid,
 			 struct btrfs_scrub_progress *progress);
 
-/*
- * The following functions are temporary exports to avoid warning on unused
- * static functions.
- */
-struct scrub_stripe;
-int queue_scrub_stripe(struct scrub_ctx *sctx,
-		       struct btrfs_block_group *bg,
-		       struct btrfs_device *dev, int mirror_num,
-		       u64 logical, u32 length, u64 physical);
-
 #endif