fsck_ufs: fix segfault with gjournal

The segfault was being hit in ckfini() (sbin/fsck_ffs/fsutil.c) while
attempting to traverse the buffer cache. The tail queue used for the
buffer cache was not initialized before dropping into gjournal_check().

Initialize the buffer cache before calling gjournal_check().

PR: 245907
Reviewed by: jhb, mckusick
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30537