The current sanity check for nilfs2 geometry information lacks checks for
the number of segments stored in superblocks, so even for device images
that have been destructively truncated or have an unusually high number of
segments, the mount operation may succeed.

This causes out-of-bounds block I/O on file system block reads or log
writes to the segments, the latter in particular causing
"a_ops->writepages" to repeatedly fail, resulting in sync_inodes_sb() to
hang.

Fix this issue by checking the number of segments stored in the superblock
and avoiding mounting devices that can cause out-of-bounds accesses.  To
eliminate the possibility of overflow when calculating the number of
blocks required for the device from the number of segments, this also adds
a helper function to calculate the upper bound on the number of segments
and inserts a check using it.

Link: https://lkml.kernel.org/r/20230526021332.3431-1-konishi.ryusuke@gmail.comSigned-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+7d50f1e54a12ba3aeae2@syzkaller.appspotmail.com
  Link: https://syzkaller.appspot.com/bug?extid=7d50f1e54a12ba3aeae2Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>