jfs: Fix shift-out-of-bounds in dbDiscardAG

When searching for the next smaller log2 block, BLKSTOL2() returned 0,
causing shift exponent -1 to be negative.

This patch fixes the issue by exiting the loop directly when negative
shift is found.

Reported-by:  <syzbot+61be3359d2ee3467e7e4@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=61be3359d2ee3467e7e4


Signed-off-by: Pei Li <peili.dev@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>