NFS: Create a common xdev_mount() function

The only difference between nfs_xdev_mount() and nfs4_xdev_mount() is the
clone_super() function called to clone the super block.  I can combine
these two functions by using the fill_super field in the mount_info
structure.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>