btrfs: scrub: add cancel/pause/removed bg checks for raid56 parity stripes

For raid56, data and parity stripes are handled differently.

For data stripes they are handled just like regular RAID1/RAID10 stripes,
going through the regular scrub_simple_mirror().

But for parity stripes we have to read out all involved data stripes and
do any needed verification and repair, then scrub the parity stripe.

This process will take a much longer time than a regular stripe, but
unlike scrub_simple_mirror(), we do not check if we should cancel/pause
or the block group is already removed.

Aligned the behavior of scrub_raid56_parity_stripe() to
scrub_simple_mirror(), by adding:

- Cancel check
- Pause check
- Removed block group check

Since those checks are the same from the scrub_simple_mirror(), also
update the comments of scrub_simple_mirror() by:

- Remove too obvious comments
  We do not need extra comments on what we're checking, it's really too
  obvious.

- Remove a stale comment about pausing
  Now the scrub is always queuing all involved stripes, and submit them
  in one go, there is no more submission part during pausing.

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

Author: adam900710

fs/btrfs/scrub.c

@@ -2091,6 +2091,20 @@ static int scrub_raid56_parity_stripe(struct scrub_ctx *sctx,
 
 	ASSERT(sctx->raid56_data_stripes);
 
+	if (atomic_read(&fs_info->scrub_cancel_req) ||
+	    atomic_read(&sctx->cancel_req))
+		return -ECANCELED;
+
+	if (atomic_read(&fs_info->scrub_pause_req))
+		scrub_blocked_if_needed(fs_info);
+
+	spin_lock(&bg->lock);
+	if (test_bit(BLOCK_GROUP_FLAG_REMOVED, &bg->runtime_flags)) {
+		spin_unlock(&bg->lock);
+		return 0;
+	}
+	spin_unlock(&bg->lock);
+
 	/*
 	 * For data stripe search, we cannot reuse the same extent/csum paths,
 	 * as the data stripe bytenr may be smaller than previous extent.  Thus
@@ -2261,18 +2275,15 @@ static int scrub_simple_mirror(struct scrub_ctx *sctx,
 		u64 found_logical = U64_MAX;
 		u64 cur_physical = physical + cur_logical - logical_start;
 
-		/* Canceled? */
 		if (atomic_read(&fs_info->scrub_cancel_req) ||
 		    atomic_read(&sctx->cancel_req)) {
 			ret = -ECANCELED;
 			break;
 		}
-		/* Paused? */
-		if (atomic_read(&fs_info->scrub_pause_req)) {
-			/* Push queued extents */
+
+		if (atomic_read(&fs_info->scrub_pause_req))
 			scrub_blocked_if_needed(fs_info);
-		}
-		/* Block group removed? */
+
 		spin_lock(&bg->lock);
 		if (test_bit(BLOCK_GROUP_FLAG_REMOVED, &bg->runtime_flags)) {
 			spin_unlock(&bg->lock);