When we do a clone swap (caused by "zfs rollback" or "zfs receive"), the ZPL
doesn't completely reload the state from the DMU; some values remain cached in
the zfsvfs_t.
steps to reproduce:
```
#!/bin/bash -x
zfs destroy -R test/fs
zfs destroy -R test/recvd
zfs create test/fs
zfs snapshot test/fs@a
zfs set userquota@$USER=1m test/fs
zfs snapshot test/fs@b
zfs send test/fs@a | zfs recv test/recvd
zfs send -i @a test/fs@b | zfs recv test/recvd
zfs userspace test/recvd
   1. should show 1m quota
      dd if=/dev/urandom of=/test/recvd/file bs=1k count=1024
      sync
      dd if=/dev/urandom of=/test/recvd/file2 bs=1k count=1024
   2. should fail with ENOSPC
      sync
      zfs unmount test/recvd
      zfs mount test/recvd
      zfs userspace test/recvd
   3. if bug above, now shows 1m quota
      dd if=/dev/urandom of=/test/recvd/file3 bs=1k count=1024
   4. if bug above, now fails with ENOSPC
```