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: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=61be3359d2ee3467e7e4
Signed-off-by: Pei Li <[e-mail]>
Signed-off-by: Dave Kleikamp <[e-mail]>

fs/jfs/jfs_dmap.c

···
 		} else if (rc == -ENOSPC) {
 			/* search for next smaller log2 block */
 			l2nb = BLKSTOL2(nblocks) - 1;
+			if (unlikely(l2nb < 0))
+				break;
 			nblocks = 1LL << l2nb;
 		} else {
 			/* Trim any already allocated blocks */