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>

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