net: Update RFS target at poll for tcp/udp

The current state of affairs is that read()/write() will setup
RFS (Receive Flow Steering) for internet protocol sockets while
poll()/epoll() does not.

When poll() gets called with a TCP or UDP socket, we should update
the flow target.

This permits to RFS (if enabled) to select the appropriate CPU for
following incoming packets.

Note: Only connected UDP sockets can benefit from RFS.

Signed-off-by: David Majnemer <majnemer@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paul Turner <pjt@google.com>
Cc: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>