Btrfs: count devices correctly in readahead during RAID 5/6 replace

Commit 5fbc7c59fd22 ("Btrfs: fix unfinished readahead thread for raid5/6
degraded mounting") fixed a problem where we would skip a missing device
when we shouldn't have because there are no other mirrors to read from
in RAID 5/6. After commit 2c8cdd6ee4e7 ("Btrfs, replace: write dirty
pages into the replace target device"), the fix doesn't work when we're
doing a missing device replace on RAID 5/6 because the replace device is
counted as a mirror so we're tricked into thinking we can safely skip
the missing device. The fix is to count only the real stripes and decide
based on that.

Signed-off-by: Omar Sandoval <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/reada.c

···
 	struct btrfs_device *prev_dev;
 	u32 blocksize;
 	u64 length;
+	int real_stripes;
 	int nzones = 0;
 	int i;
 	unsigned long index = logical >> PAGE_CACHE_SHIFT;
···
 		goto error;
 	}
 
-	for (nzones = 0; nzones < bbio->num_stripes; ++nzones) {
+	real_stripes = bbio->num_stripes - bbio->num_tgtdevs;
+	for (nzones = 0; nzones < real_stripes; ++nzones) {
 		struct reada_zone *zone;
 
 		dev = bbio->stripes[nzones].dev;