btrfs: raid56: simplify tracking of Q stripe presence

There are temporary variables tracking the index of P and Q stripes, but
none of them is really used as such, merely for determining if the Q
stripe is present. This leads to compiler warnings with
-Wunused-but-set-variable and has been reported several times.

fs/btrfs/raid56.c: In function ‘finish_rmw’:
fs/btrfs/raid56.c:1199:6: warning: variable ‘p_stripe’ set but not used [-Wunused-but-set-variable]
 1199 |  int p_stripe = -1;
      |      ^~~~~~~~
fs/btrfs/raid56.c: In function ‘finish_parity_scrub’:
fs/btrfs/raid56.c:2356:6: warning: variable ‘p_stripe’ set but not used [-Wunused-but-set-variable]
 2356 |  int p_stripe = -1;
      |      ^~~~~~~~

Replace the two variables with one that has a clear meaning and also get
rid of the warnings. The logic that verifies that there are only 2
valid cases is unchanged.
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -1196,22 +1196,19 @@ static noinline void finish_rmw(struct btrfs_raid_bio *rbio)
 	int nr_data = rbio->nr_data;
 	int stripe;
 	int pagenr;
-	int p_stripe = -1;
-	int q_stripe = -1;
+	bool has_qstripe;
 	struct bio_list bio_list;
 	struct bio *bio;
 	int ret;
 
 	bio_list_init(&bio_list);
 
-	if (rbio->real_stripes - rbio->nr_data == 1) {
-		p_stripe = rbio->real_stripes - 1;
-	} else if (rbio->real_stripes - rbio->nr_data == 2) {
-		p_stripe = rbio->real_stripes - 2;
-		q_stripe = rbio->real_stripes - 1;
-	} else {
+	if (rbio->real_stripes - rbio->nr_data == 1)
+		has_qstripe = false;
+	else if (rbio->real_stripes - rbio->nr_data == 2)
+		has_qstripe = true;
+	else
 		BUG();
-	}
 
 	/* at this point we either have a full stripe,
 	 * or we've read the full stripe from the drive.
@@ -1255,7 +1252,7 @@ static noinline void finish_rmw(struct btrfs_raid_bio *rbio)
 		SetPageUptodate(p);
 		pointers[stripe++] = kmap(p);
 
-		if (q_stripe != -1) {
+		if (has_qstripe) {
 
 			/*
 			 * raid6, add the qstripe and call the
@@ -2353,8 +2350,7 @@ static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
 	int nr_data = rbio->nr_data;
 	int stripe;
 	int pagenr;
-	int p_stripe = -1;
-	int q_stripe = -1;
+	bool has_qstripe;
 	struct page *p_page = NULL;
 	struct page *q_page = NULL;
 	struct bio_list bio_list;
@@ -2364,14 +2360,12 @@ static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
 
 	bio_list_init(&bio_list);
 
-	if (rbio->real_stripes - rbio->nr_data == 1) {
-		p_stripe = rbio->real_stripes - 1;
-	} else if (rbio->real_stripes - rbio->nr_data == 2) {
-		p_stripe = rbio->real_stripes - 2;
-		q_stripe = rbio->real_stripes - 1;
-	} else {
+	if (rbio->real_stripes - rbio->nr_data == 1)
+		has_qstripe = false;
+	else if (rbio->real_stripes - rbio->nr_data == 2)
+		has_qstripe = true;
+	else
 		BUG();
-	}
 
 	if (bbio->num_tgtdevs && bbio->tgtdev_map[rbio->scrubp]) {
 		is_replace = 1;
@@ -2393,7 +2387,7 @@ static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
 		goto cleanup;
 	SetPageUptodate(p_page);
 
-	if (q_stripe != -1) {
+	if (has_qstripe) {
 		q_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
 		if (!q_page) {
 			__free_page(p_page);
@@ -2416,8 +2410,7 @@ static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
 		/* then add the parity stripe */
 		pointers[stripe++] = kmap(p_page);
 
-		if (q_stripe != -1) {
-
+		if (has_qstripe) {
 			/*
 			 * raid6, add the qstripe and call the
 			 * library function to fill in our p/q