Btrfs-progs: Initialize stripesize to the value of sectorsize

stripesize should ideally be set to the value of sectorsize. However
previous versions of btrfs-progs/mkfs.btrfs had set stripesize to a
value of 4096. On machines with PAGE_SIZE other than 4096, This could
lead to the following scenario,

- /dev/loop0, /dev/loop1 and /dev/loop2 are mounted as a single
  filesystem. The filesystem was created by an older version of mkfs.btrfs
  which set stripesize to 4k.
- losetup -a
   /dev/loop0: [0030]:19477 (/root/disk-imgs/file-0.img)
   /dev/loop1: [0030]:16577 (/root/disk-imgs/file-1.img)
   /dev/loop2: [64770]:3423229 (/root/disk-imgs/file-2.img)
- /etc/mtab lists only /dev/loop0
- losetup /dev/loop4 /root/disk-imgs/file-1.img
  The new mkfs.btrfs invoked as 'mkfs.btrfs -f /dev/loop4' succeeds even
  though /dev/loop1 has already been mounted and has
  /root/disk-imgs/file-1.img as its backing file.

The above behaviour occurs because check_super() function returns an
error code (due to stripesize not being set to 4096) and hence
check_mounted_where() function treats /dev/loop1 as a disk containing a
filesystem other than Btrfs.

Hence as a workaround this commit allows 4096 as a valid stripesize.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: David Sterba <dsterba@suse.com>

disk-io.c

@@ -1476,7 +1476,8 @@ static int check_super(struct btrfs_super_block *sb)
 		error("invalid bytes_used %llu", btrfs_super_bytes_used(sb));
 		goto error_out;
 	}
-	if (btrfs_super_stripesize(sb) != 4096) {
+	if ((btrfs_super_stripesize(sb) != 4096)
+		&& (btrfs_super_stripesize(sb) != btrfs_super_sectorsize(sb))) {
 		error("invalid stripesize %u", btrfs_super_stripesize(sb));
 		goto error_out;
 	}

mkfs.c

@@ -1482,6 +1482,7 @@ int main(int argc, char **argv)
 	}
 
 	sectorsize = max(sectorsize, (u32)sysconf(_SC_PAGESIZE));
+	stripesize = sectorsize;
 	saved_optind = optind;
 	dev_cnt = argc - optind;
 	if (dev_cnt == 0)