jfs: nlink overflow in jfs_rename

jfs: nlink overflow in jfs_rename

If nlink is maximal for a directory (-1) and inside that directory you
perform a rename for some child directory (not moving from the parent),
then the nlink of the first directory is first incremented and later
decremented. Normally this is fine, but when nlink = -1 this causes a
wrap around to 0, and then drop_nlink issues a warning.

After applying the patch syzbot no longer issues any warnings. I also
ran some basic fs tests to look for any regressions.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Reported-by: syzbot+9131ddfd7870623b719f@syzkaller.appspotmail.com
Closes: https://syzbot.org/bug?extid=9131ddfd7870623b719f
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>