staging/lustre/llite: Anonymous dentry incorrectly identified as root

When exporting Lustre via NFS on SLES11SP2, this check incorrectly identifies
anonymous dentries as root dentries.  This is due to a change in SLES11SP2
which makes the d_name.name for anonymous dentries the same as that for root
dentries.  (Details in LU-3484.)

This changes the check to directly compare the value of the dentry pointer
to the root dentry pointer found in the superblock, rather than using the name.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3484
Lustre-change: http://review.whamcloud.com/6726


Signed-off-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Alexey Shvetsov <alexxy@gentoo.org>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>