reiserfs: add check for invalid 1st journal block

syzbot reported divide error in reiserfs.
The problem was in incorrect journal 1st block.

Syzbot's reproducer manualy generated wrong superblock
with incorrect 1st block. In journal_init() wasn't
any checks about this particular case.

For example, if 1st journal block is before superblock
1st block, it can cause zeroing important superblock members
in do_journal_end().

Link: https://lore.kernel.org/r/20210517121545.29645-1-paskripkin@gmail.com


Reported-by:  <syzbot+0ba9909df31c6a36974d@syzkaller.appspotmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>