btrfs-progs: convert: Fix migrate_super_block() to work with 64k sectorsize

btrfs-progs: convert: Fix migrate_super_block() to work with 64k sectorsize

migrate_super_block() uses sectorsize to refer to the size of the
superblock. Hence on 64k sectorsize filesystems, it ends up computing
checksum beyond the super block length (i.e.
BTRFS_SUPER_INFO_SIZE). This commit fixes the bug by using
BTRFS_SUPER_INFO_SIZE instead of sectorsize of the underlying
filesystem.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>