RHEL6: After upgrading the kernel, umount of NFS exported filesystem returns EBUSY even after stopping NFS and fuser and lsof shows no open files

Issue

After upgrading kernel version to 2.6.32-696.16.1.el6 a umount of an NFS exported filesystem returns EBUSY even after stopping NFS and there are no open files shown with lsof or fuser. After downgrading to older kernel version 2.6.32-696.10.3.el6_86_64, this issue does not reproduce:

# service nfs status
rpc.svcgssd is stopped
rpc.mountd is stopped
nfsd is stopped
rpc.rquotad is stopped
# fuser /export
# lsof /export
# unmount /export
unmount: /export: device is busy.
# umount -f /export
umount2: Device or resource busy
umount: /export: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
umount2: Device or resource busy
# exportfs -au
# umount -f /export
umount2: Device or resource busy
umount: /export: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
umount2: Device or resource busy