SUNRPC: Don't auto-disconnect from the local rpcbind socket

There is no need for the kernel to time out the AF_LOCAL connection to
the rpcbind socket, and doing so is problematic because when it is
time to reconnect, our process may no longer be using the same mount
namespace.

Reported-by: Nix <nix@esperi.org.uk>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Jeff Layton <jlayton@redhat.com>
Cc: stable@vger.kernel.org # 3.9.x