Btrfs: add orphan before truncating pagecache

Running xfstests 83 in a loop would sometimes fail the fsck.  This happens
because if we invalidate a page that already has an ordered extent setup for
it we will complete the ordered extent ourselves, assuming that the truncate
will clean everything up.  The problem with this is there is plenty of time
for the truncate to fail after we've done this work.  So to fix this we need
to add the orphan item first to make sure the cleanup gets done properly,
and then we can truncate the pagecache and all that stuff and be safe.  This
fixes the btrfsck failures I was seeing while running 83 in a loop.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>