Btrfs-progs: reset balance before reiniting extent root

Btrfs-progs: reset balance before reiniting extent root

When we re-init the extent root we make it completely empty, so when we reset a
pending balance we will fail to find refs for any blocks we may cow, which will
result in errors and we will exit out.  We need to reset the balance first so
the normal cow stuff doesn't freak out and then we can re-init the extent tree.
Thanks,

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