NFS: Add a helper to remove case-insensitive aliases

When dealing with case insensitive names, the client has no idea how the
server performs the mapping, so cannot collapse the dentries into a
single representative. So both rename and unlink need to deal with the
fact that there could be several dentries representing the file, and
have to somehow force them to be revalidated. Use d_prune_aliases() as a
big hammer approach.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>