unmount partition error after upgrading to new kernel version 2.6.32-696.16.1.el6

After upgrading kernel version to 2.6.32-696.16.1.el6

Following error message shows:
unmount /usr
unmount /usr:device is busy

unmount /export/home
unmount: /export/home: device is busy.

We observed this with other types of partitions as well :

[Host1 ~]# umount /tmp/
umount: /tmp: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
TYPE causing error :ext4

However, if we mount a File System of type vfat then it is easily unmounted without any errors :

[Host1 ~]# df -T /mnt
Filesystem Type 1K-blocks Used Available Use% Mounted on
/dev/sda1 vfat 204580 260 204320 1% /mnt
[Host1 ~]# umount /mnt
[Host1 ~]#

There is something which is holding the resources and thus it cant be unmounted successfully. However, it displays nothing in lsof and fuser output for these partitions :

[Host1]# umount /nz
umount: /nz: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
[Host1]#

We see that there is no process is holding up for /nz partition
[Host1 ~]# lsof /nz

We rae not sure what is causing this behavior. Could anyone please help us on this?

Thanks,
Ruchi