jfs: fix array-index-out-of-bounds in diAlloc

Currently there is not check against the agno of the iag while
allocating new inodes to avoid fragmentation problem. Added the check
which is required.

Reported-by: syzbot+79d792676d8ac050949f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=79d792676d8ac050949f
Signed-off-by: Manas Ghandat <ghandatmanas@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

Author: manasghandat

fs/jfs/jfs_imap.c

@@ -1320,7 +1320,7 @@ diInitInode(struct inode *ip, int iagno, int ino, int extno, struct iag * iagp)
 int diAlloc(struct inode *pip, bool dir, struct inode *ip)
 {
 	int rc, ino, iagno, addext, extno, bitno, sword;
-	int nwords, rem, i, agno;
+	int nwords, rem, i, agno, dn_numag;
 	u32 mask, inosmap, extsmap;
 	struct inode *ipimap;
 	struct metapage *mp;
@@ -1356,6 +1356,9 @@ int diAlloc(struct inode *pip, bool dir, struct inode *ip)
 
 	/* get the ag number of this iag */
 	agno = BLKTOAG(JFS_IP(pip)->agstart, JFS_SBI(pip->i_sb));
+	dn_numag = JFS_SBI(pip->i_sb)->bmap->db_numag;
+	if (agno < 0 || agno > dn_numag)
+		return -EIO;
 
 	if (atomic_read(&JFS_SBI(pip->i_sb)->bmap->db_active[agno])) {
 		/*