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