nfsd: make reclaim_str_hashtbl allocated per net

This hash holds nfs4_clients info, which are network namespace aware.
So let's make it allocated per network namespace.

Note: this hash is used only by legacy tracker. So let's allocate hash in
tracker init.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>