NFS: nfs_prime_dcache must validate the filename

Before we try to stash it in the dcache, we need to at least check
that the filename passed to us by the server is non-empty and doesn't
contain any illegal '\0' or '/' characters.

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