Do not allow snapshots on UFS filesystems using gjournal.

The gjournal implementation does not properly handle the freeing
of blocks that may be part of a snapshot. Adding this support to
gjournal would require considerable effort. For now we simply
do not allow snapshots to be taken on filesystems using gjournal.

Reported by: ant_mail@inbox.ru
PR: 280216
MFC after: 1 week