Btrfs-progs: add an option to btrfs-image to walk the trees

Btrfs-progs: add an option to btrfs-image to walk the trees

When working with a user with a broken file system I noticed I wasn't able to
read some of the blocks properly from the restored image.  This is because his
extent tree was corrupt and was missing references to some of the blocks, which
means they weren't copied into the image when he generated it.  So add a -w
option which will walk all of the trees manually and copy them into the image.
This way we can run fsck against a complete file system image and fix any bugs
in fsck.  Thanks,

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