Btrfs: reset ret in record_one_backref

I was getting warnings when running find ./ -type f -exec btrfs fi defrag -f {}
\; from record_one_backref because ret was set.  Turns out it was because it was
set to 1 because the search slot didn't come out exact and we never reset it.
So reset it to 0 right after the search so we don't leak this and get
uneccessary warnings.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>