NFS: Use the dentry superblock directly in nfs_statfs()

Use the nominated dentry's superblock directly in the NFS statfs() op to get a
file handle, rather than using s_root (which will become a dummy dentry in a
future patch).

Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>