journal: remove an obsolete member from struct journal_descriptor

The member epoch in struct journal_descriptor is obsolete. Let's
remove this.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
journal: call panic() when the journaling file is broken

This change makes the journaling subsystem more defensive and enhances the error
logging.

Cc: Kees Bos <k.bos@capitar.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>