Btrfs: don't panic if we get an error while balancing V2

A user reported an error where if we try to balance an fs after a device has
been removed it will blow up.  This is because we get an EIO back and this is
where BUG_ON(ret) bites us in the ass.  To fix we just exit.  Thanks,

Reported-by: Anand Jain <Anand.Jain@oracle.com>
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>