jfs: fix shift-out-of-bounds in dbJoin

[ Upstream commit cca974da ]

Currently while joining the leaf in a buddy system there is shift out
of bound error in calculation of BUDSIZE. Added the required check
to the BUDSIZE and fixed the documentation as well.

Reported-by:  <syzbot+411debe54d318eaed386@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=411debe54d318eaed386


Signed-off-by: Manas Ghandat <ghandatmanas@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>