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

[ Upstream commit a5f5e4698f8abbb25fe4959814093fb5bfa1aa9d ]

When dmt_budmin is less than zero, it causes errors
in the later stages. Added a check to return an error beforehand
in dbAllocCtl itself.

Reported-by:  <syzbot+b5ca8a249162c4b9a7d0@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=b5ca8a249162c4b9a7d0


Signed-off-by: Ghanshyam Agrawal <ghanshyam1898@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>