Test case looks like this:
```
mdconfig -a -t swap -s 32m
newfs -U /dev/md0
mount /dev/md0 /mnt
touch /mnt/foo
setextattr user a aaaa /mnt/foo
setextattr user b bbbb /mnt/foo
setextattr user c cccc /mnt/foo
rmextattr user c /mnt/foo
lsextattr user /mnt/foo
getextattr user a /mnt/foo
getextattr user b /mnt/foo
```