nfsd: nfsd_file cache entries should be per net namespace

Ensure that we can safely clear out the file cache entries when the
nfs server is shut down on a container. Otherwise, the file cache
may end up pinning the mounts.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>