btrfs: calculate physical_end using dev_extent_len directly in scrub_stripe()

The variable @physical_end is the exclusive stripe end, currently it's
calculated using @physical + @dev_extent_len / map->stripe_len *
 map->stripe_len.

And since at allocation time we ensured dev_extent_len is stripe_len
aligned, the result is the same as @physical + @dev_extent_len.

So this patch will just assign @physical and @physical_end early,
without using @nstripes.

This is especially helpful for any possible out: label user, as now we
only need to initialize @offset before going to out: label.

Since we're here, also make @physical_end constant.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -3160,10 +3160,10 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	int slot;
 	u64 nstripes;
 	struct extent_buffer *l;
-	u64 physical;
+	u64 physical = map->stripes[stripe_index].physical;
 	u64 logical;
 	u64 logic_end;
-	u64 physical_end;
+	const u64 physical_end = physical + dev_extent_len;
 	u64 generation;
 	int mirror_num;
 	struct btrfs_key key;
@@ -3182,7 +3182,6 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	int extent_mirror_num;
 	int stop_loop = 0;
 
-	physical = map->stripes[stripe_index].physical;
 	offset = 0;
 	nstripes = div64_u64(dev_extent_len, map->stripe_len);
 	mirror_num = 1;
@@ -3219,7 +3218,6 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	path->reada = READA_FORWARD;
 
 	logical = chunk_logical + offset;
-	physical_end = physical + nstripes * map->stripe_len;
 	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
 		get_raid56_logic_offset(physical_end, stripe_index,
 					map, &logic_end, NULL);