Btrfs: abort the transaction when we don't find our extent ref

I'm not sure why we weren't aborting here in the first place, it is obviously a
bad time from the fact that we print the leaf and yell loudly about it.  Fix
this up, otherwise we panic because our path could be pointing into oblivion.
Thanks,

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