Btrfs: fix return code in drop_objectid_items

So dpkg fsync()'s the file and the directory containing the file whenever it
writes to a file which is really slow in btrfs.  This is partly because
fsync()'ing a directory _always_ committed the transaction instead of just
going to the tree log.  This is because drop_objectid_items() would return 1
since it does a btrfs_search_slot() which returns 1.  In tree-log jargon
this means that we have to commit the transaction to be safe.  So just check
if ret is greater than 0 and set it to 0 if it does.  With this patch we now
use the tree-log instead of committing the entire transaction, which is
twice as fast on my box.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>