btrfs: factor out stripe length calculation into a helper

Currently there are two locations which need to calculate the real
length of a stripe (which can be at the end of a chunk, and the chunk
size may not always be 64K aligned).

Factor them into a helper as we're going to have a third user soon.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
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.c

@@ -1648,14 +1648,20 @@ static void scrub_reset_stripe(struct scrub_stripe *stripe)
 	}
 }
 
+static u32 stripe_length(const struct scrub_stripe *stripe)
+{
+	ASSERT(stripe->bg);
+
+	return min(BTRFS_STRIPE_LEN,
+		   stripe->bg->start + stripe->bg->length - stripe->logical);
+}
+
 static void scrub_submit_extent_sector_read(struct scrub_ctx *sctx,
 					    struct scrub_stripe *stripe)
 {
 	struct btrfs_fs_info *fs_info = stripe->bg->fs_info;
 	struct btrfs_bio *bbio = NULL;
-	unsigned int nr_sectors = min(BTRFS_STRIPE_LEN, stripe->bg->start +
-				      stripe->bg->length - stripe->logical) >>
-				  fs_info->sectorsize_bits;
+	unsigned int nr_sectors = stripe_length(stripe) >> fs_info->sectorsize_bits;
 	u64 stripe_len = BTRFS_STRIPE_LEN;
 	int mirror = stripe->mirror_num;
 	int i;
@@ -1729,9 +1735,7 @@ static void scrub_submit_initial_read(struct scrub_ctx *sctx,
 {
 	struct btrfs_fs_info *fs_info = sctx->fs_info;
 	struct btrfs_bio *bbio;
-	unsigned int nr_sectors = min(BTRFS_STRIPE_LEN, stripe->bg->start +
-				      stripe->bg->length - stripe->logical) >>
-				  fs_info->sectorsize_bits;
+	unsigned int nr_sectors = stripe_length(stripe) >> fs_info->sectorsize_bits;
 	int mirror = stripe->mirror_num;
 
 	ASSERT(stripe->bg);