btrfs: reduce struct btrfs_fs_devices size by moving fsid_change

Pack bool fsid_change and bool seeding with other bool declarations in the
struct btrfs_fs_devices, approximately 6 bytes is saved, depending on
the config.

   before: 512 bytes
   after: 496 bytes

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>