Fix fsck_ufs segfaults with gjournal (SU+J)

The segfault was being hit in ckfini() (sbin/fsck_ffs/fsutil.c)
while attempting to traverse the buffer cache to flush dirty buffers.
The tail queue used for the buffer cache was not initialized before
dropping into gjournal_check(). Move the buffer initialization earlier
so that it has been done before calling gjournal_check().

Reported by: crypt47, nvass
Fix by: Robert Wing
Tested by: Robert Wing
PR: 255030
PR: 255979
MFC after: 3 days
Sponsored by: Netflix