Btrfs-progs: read super properly in btrfs-image

Btrfs-progs: read super properly in btrfs-image

When btrfs-image makes a metadump it'll map all the blocks from their logical
address to their physical.  This works out fine with the exception of the super
block, which is the physical offset.  Normally this just works, but if the user
has balanced their fs it'll either crash btrfs-image or it'll copy some
completely arbitrary data.  This forces btrfs-image to read the super directly
from the disk.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>