[TCP]: Process linger2 timeout consistently.

Based upon guidance from Alexey Kuznetsov.

When linger2 is active, we check to see if the fin_wait2
timeout is longer than the timewait.  If it is, we schedule
the keepalive timer for the difference between the timewait
timeout and the fin_wait2 timeout.

When this orphan socket is seen by tcp_keepalive_timer()
it will try to transform this fin_wait2 socket into a
fin_wait2 mini-socket, again if linger2 is active.

Not all paths were setting this initial keepalive timer correctly.
The tcp input path was doing it correctly, but tcp_close() wasn't,
potentially making the socket linger longer than it really needs to.

Signed-off-by: David S. Miller <davem@davemloft.net>