LU-1718 client: Restore NFS export for Lustre on 3.X kernels

LU-1718 client: Restore NFS export for Lustre on 3.X kernels

In Linux 3.0+ kernels struct file_system_type changed the
get_sb function to a new function called mount which was
different in that the vfsmount data was no longer passed in.
The vfsmount data was used by the llite layer for nfs export
function called get_name to search for filp that was then used
with the ll_readdir method. The approach to solve this change
was to go the route of btrfs and gfs2 to refactor some of the
llite methods to implement a directory scan independent of
filp which could be shared with nfs export funtionality.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I72730476b120cec1ede6e03c774c9e470a1a5a70
Reviewed-on: http://review.whamcloud.com/3624
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>