btrfs: enhance superblock checks

The superblock checksum is not verified upon mount. <awkward silence>

Add that check and also reorder existing checks to a more logical
order.

Current mkfs.btrfs does not calculate the correct checksum of
super_block and thus a freshly created filesytem will fail to mount when
this patch is applied.

First transaction commit calculates correct superblock checksum and
saves it to disk.

Reproducer:
$ mfks.btrfs /dev/sda
$ mount /dev/sda /mnt
$ btrfs scrub start /mnt
$ sleep 5
$ btrfs scrub status /mnt
... super:2 ...

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>