xfs: use null daddr for unset first bad log block

xfs: use null daddr for unset first bad log block

xlog_do_recovery_pass() may return before setting first_bad.  The caller
must distinguish that case from an error at a valid log block, including
block zero after the log wraps.

Initialize first_bad to XFS_BUF_DADDR_NULL and test it explicitly before
treating the error as a torn write.

Fixes: 7088c4136fa1 ("xfs: detect and trim torn writes during log recovery")
Suggested-by: Darrick J. Wong <djwong@kernel.org>
Reported-by: syzbot+b7dfbed0c6c2b5e9fd34@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b7dfbed0c6c2b5e9fd34
Cc: stable@vger.kernel.org # v4.5
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>