fs/jfs: fix shift exponent db_agl2size negative

As a shift exponent, db_agl2size can not be less than 0. Add the missing
check to fix the shift-out-of-bounds bug reported by syzkaller:

 UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:2227:15
 shift exponent -744642816 is negative

Reported-by: syzbot+0be96567042453c0c820@syzkaller.appspotmail.com
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

fs/jfs/jfs_dmap.c

@@ -193,7 +193,8 @@ int dbMount(struct inode *ipbmap)
 	bmp->db_agwidth = le32_to_cpu(dbmp_le->dn_agwidth);
 	bmp->db_agstart = le32_to_cpu(dbmp_le->dn_agstart);
 	bmp->db_agl2size = le32_to_cpu(dbmp_le->dn_agl2size);
-	if (bmp->db_agl2size > L2MAXL2SIZE - L2MAXAG) {
+	if (bmp->db_agl2size > L2MAXL2SIZE - L2MAXAG ||
+	    bmp->db_agl2size < 0) {
 		err = -EINVAL;
 		goto err_release_metapage;
 	}